Return Flow System for Ion Concentration Polarization (ICP)

ABSTRACT

A device for purifying and/or concentrating a first water stream containing ionic impurities includes a first and second ion exchange membrane and a first porous membrane. The ion exchange membranes have the same charge, and a channel into which the first water stream can be directed is defined between the first and second ion exchange membranes. The channel has an inlet end and a return flow end and comprises a first and second outlet. The inlet and at least the first outlet are located on the inlet end of the channel and are separated by the first porous membrane that traverses the length of the channel between the ion exchange membranes and terminates at a return flow zone that is at least partially closed. At least part of the first water stream flows through the first porous membrane, joining a first return flow stream.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/839,152, filed on 3 Apr. 2020, which was a continuation ofInternational Application No. PCT/US19/14941, which designated theUnited States and was filed on 24 Jan. 2019, which claims the benefit ofU.S. Provisional Application No. 62/621,839, filed on 25 Jan. 2018. Theentire teachings of the above-referenced applications are incorporatedherein by reference.

BACKGROUND

Ion concentration polarization (ICP) desalination and trifurcate ICPdesalination systems have been described, for example, in U.S. PatentApp. Pub. No. 2014/0374274 A1 (entitled “Water Desalination/Purificationand Bio-Agent Preconcentration by Ion Concentration Polarization”) andU.S. Patent App. Pub. No. 2016/0115045 A1 (entitled “Purification ofUltra-High Saline and Contaminated Water by Multi-Stage IonConcentration Polarization (ICP) Desalination”). As described in thesepatent publications, in ICP desalination, both dilute and concentratestreams are separately acquired between two identical ion exchangemembranes (IEM). In contrast, conventional electrodialysis (ED) requiresalternating different IEMs, for example, alternating an anion exchangemembrane (AEM) and a cation exchange membrane (CEM).

It has been reported that ICP utilizing CEMs can enhance salt removalratio up to 20% compared to electrodialysis under constant currentapplied, along with other advantages as compared with relatedelectrodialysis techniques (Kim et al. (2016), Scientific Reports6:31850; doi: 10.1038/srep31850). To improve the energy efficiency ofICP, the trifurcate ICP desalination system and method were developed;it splits the feed stream into three different output flows according toconcentration distribution between membranes. The trifurcate ICP enablesthe collection of thin ion-depleted and ion-enriched layers that developnext to the IEM surface while the majority of the fluid is in the middleof the channel by dividing outlets of target stream within one channelunit.

However, even with the trifurcate ICP desalination system, a highcurrent density, corresponding to an over-limiting current, needs to beapplied to provide highly desalted and concentrated water streams. Thisover-limiting current is accompanied by two phenomena, chaoticelectroconvection in the dilute stream and propagation of enriched saltfrom the concentrate stream, resulting in increased energy consumption.The electroconvection in the dilute stream generates a chaotic flowmotion which increases energy dissipation and causes undesirable flowmixing. The highly enriched salt in the concentrate stream, in turn,propagates to the dilute stream, causing a decline in salt removalefficiency. Since the dilute and concentrate streams are on the samechannel component, they can affect each other without any restrictions.

Systems and methods described herein are based on the recognition thattwo strategies can be used to improve the energy efficiency ofdesalination and/or salt production. The strategies provide a channelstructure that minimizes chaotic electroconvection at the dilute streamand/or that blocks enriched salt propagation from the concentrate stream

SUMMARY

Described herein are return flow ICP and ED systems and methods that canbe used for water desalination and/or concentration of a wide range oftarget brine and other aqueous and contaminated streams. Specifically, anewly designed flow pathway incorporating a porous membrane has beendeveloped, a so-called return flow ICP desalination/concentrationsystem, which suppresses chaotic electroconvection in the dilute streamand suppresses or prevents highly enriched salt propagation from theconcentrate stream (described, for example, in more detail in FIGS. 3-7). The systems can be characterized by a primary channel defined byopposing ion exchange membranes with an inlet at one end and one or morereturn flow channels disposed therein, an anode, and a cathodeconfigured to create an electric field across the channel. The one ormore return flow channels can be defined by one of the ion exchangemembranes and a porous membrane that extends parallel (or approximatelyparallel) thereto. The return flow channel is configured within theprimary channel to allow a feed stream to enter the channel through aninlet, flow along the primary channel to the distal end of the channel,and at least a portion of the feed stream (that is either enriched ordepleted in ions) to flow into the return flow channel(s) and backtowards the inlet end of the channel, allowing cross-current flow acrossthe porous membrane. The systems and methods described herein utilize aporous membrane installed between different streams as a physical flowseparation structure, resulting in a flow barrier. The porous membraneallows fluid to flow partially by a pressure difference but also allowsions to freely pass through. The partial fluid that flows through theporous membrane (“porous membrane flow” or “PM-flow”) generates a flowbarrier that acts as a suppressor (of chaotic electroconvection) for thedilute stream and a preventer (of highly enriched salt propagation) forthe concentrate stream. In addition, the systems and methods describedherein feature an inlet for the feed stream and an outlet for a targetstream (the dilute/purified stream or the concentrate stream) next toeach other, but the inlet and the outlet are separated from each otherby the porous membrane, which runs the length of the channel, except atthe end of channel providing for return-flow, for maximizing pressuredifference. This configuration allows a maximized flow through theporous membrane that results in a flow barrier and the return-flow,which has the effect of sweeping a mass on the IEM surfaces by shearstress. The newly designed channel pathway can also result in anincrease in the traveling length of the stream. As described in moredetail below, the fluid, in some configurations, effectively passes thechannel twice (by return flow), effectively increasing the length of thechannel (feedwater dwell time). In other configurations, one of thestreams (the diluate stream or the concentrate stream) effectivelypasses the channel twice, increasing the feed water dwell time.Meanwhile, the other stream (the concentrate stream or the diluatestream, respectively) flows out (to its outlet) without any dwell timeincrease. The systems entailing the use of the return-flow system can beapplied for water desalination and/or concentration for a wide range oftarget brine and other target streams.

A device for purifying and/or concentrating a first water streamcontaining charged contaminants can include a first ion exchangemembrane and a second ion exchange membrane. The ion exchange membranesare characterized by the same charge, and a channel into which the firstwater stream can be directed is defined between the first ion exchangemembrane and the second ion exchange membrane. The channel is furthercharacterized as having an inlet end and a return flow end, wherein theinlet end is where an inlet is located, and wherein the return flow endis downstream from the inlet end when the first water stream is directedinto the channel through the inlet. The device also includes a firstporous membrane. The channel further comprises a first outlet and asecond outlet. The inlet and at least the first outlet are located onthe inlet end of the channel and are separated by the first porousmembrane that traverses the length of the channel between the ionexchange membranes and terminates at a return flow zone. The return flowzone is a section of the channel at the return flow end, and the returnflow end is at least partially closed. The ion exchange membranes andthe first porous membrane are configured such that directing the firstwater stream into the inlet of the channel and applying an electricfield across the channel forms an ion depletion zone comprising apurified water stream and an ion enrichment zone comprising aconcentrated ion aqueous stream, wherein at least part of the firstwater stream enters the return flow zone and forms a first return flowstream that flows to the opposing side of the first porous membrane andthe first return flow stream flows in the direction of the first outlet,and at least part of the first water stream, including the water and thecharged contaminants, adjacent to the first porous membrane flowsthrough the first porous membrane joining the first return flow stream.The purified water stream is the stream directed to the first or thesecond outlet, and the concentrated-charged-contaminant aqueous streamis the stream directed to the other of the first and the second outlet.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of embodiments of the invention, as illustrated in theaccompanying drawings in which like reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIGS. 1A, 1B, and 1C are schematics. FIGS. 1A and 1B are schematicsshowing standard bipolar ED versus unipolar ICP desalination. FIG. 1Ashows standard bipolar ED, and FIG. 1B shows unipolar ICP platforms havedesalted flows with low ion concentration at the anodic side of CEMs andat the cathodic side of AEMs (white regions); and vice versa for brineflows (dark gray regions). An ICP platform can also be built with AEMsbut the location of desalted/brine flows would be reversed. FIG. 1C is aschematic showing depletion on CEM or AEM. In FIG. 1C, the arrowsindicate the ion flux through the membranes. The black dotted boxes arethe one membrane pair (N=1) for ED and ICP systems, which are repeated;both are functionally matched (membrane number, flow rate, waterrecovery, etc.).

FIG. 2A is a schematic showing trifurcated ICP desalination. To obtain athin depletion stream and a small amount of dilute flow with highpurity, one can trifurcate the main channel into three different outputflows in accordance with the concentration distribution betweenmembranes. The intermediate stream (the middle stream) can be fed to thenext stage by a batch process or recirculation. FIG. 2B is a fluorescentimage of trifurcated ICP desalination using sodium chloride solution(4V).

FIG. 3 is a schematic illustration of a desalination applicationcomprising return-flow using the “counter flow” ICP (CF-ICP) system thatresults in a suppressive flow barrier for suppressing a chaoticelectroconvection in the dilute stream. “α+” and “β−” indicate cationand anion movement, respectively, by application of an electric field.

FIG. 4 is a schematic illustration of a concentration application usingthe CF-ICP system that results in a preventive flow barrier forpreventing the propagation of highly enriched mass in the concentratestream.

FIG. 5 is a schematic illustration of a desalination/concentrationapplication using the return flow ICP (RF-ICP) system resulting inunited (suppressing chaotic electroconvection in the dilute stream andpreventing propagation of highly enriched mass in the concentratestream) flow barriers.

FIG. 6 is a schematic illustration showing double flow barriers with areturn-flow system for a conventional electrodialysis.

FIG. 7A is a schematic illustration of ion transport and flow path inreturn-flow ICP (RF-ICP) desalination. The solid lines (top and bottom,respectively) indicate the depletion and concentration boundary layers,respectively. FIG. 7B shows the distribution of current density and thethickness of the depletion layer along the membrane expressed as anarbitrary value.

FIG. 8A is a schematic illustration of ion transport and flow path incounter-flow ICP (CF-ICP) desalination. The solid lines (top and bottom,respectively) indicate depletion and concentration boundary layers,respectively. FIG. 8B shows concentration profiles for RF-ICP and CF-ICPalong CEM. The top and bottom lines indicate concentration profiles ofthe concentration and depletion boundary layers, respectively.

FIGS. 9A-9D are schematic illustrations comparing Bi-ICP (FIG. 9A),Tri-ICP (FIG. 9B), RF-ICP (FIG. 9C) and CF-ICP (FIG. 9D).

FIGS. 10A-10C shows power consumption according to salt removal ratio(SRR) with feed concentrations of 70 (FIG. 10A), 100 (FIG. 10B), and 160kppm (FIG. 10C). (ICP desalination system with 30 cm of effectivemembrane length. 0.5 mm/s of flow velocity.)

FIGS. 11A-11D are schematic illustrations of ion transport and flow pathin electrodialysis (ED) (FIG. 11A), bifurcate ion concentrationpolarization (Bi-ICP) (FIG. 11B), trifurcate ICP (Tri-ICP) (FIG. 11C)and return-flow ICP (RF-ICP) (FIG. 11D) desalination. Arrows (“α+” and“β−”) respectively indicate the cation and anion movement by applicationof an electric field. Shading intensity represents ion concentration.

FIG. 11E Section C and FIG. 11F Section D, respectively, represent thedetails of the flow path and concentration profile. The solid lines, topand bottom, indicate the depletion and concentration boundary layers,respectively.

FIG. 11G plots a distribution of current density along the CEM and theconcentration profile near the CEM. The lines that gradually decreasemoving left to right along the horizontal axis indicate current densitydistributions along the CEM, and the lines that increase moving left toright along the horizontal axis indicate concentration profiles (C₀)near the CEM. Solid lines and dotted lines indicate Tri-ICP and RF-ICP,respectively, for FIGS. 11E, 11F, and 11G.

FIG. 12 is a schematic of a measurement system for an analyticalexperiment using a RF-ICP desalination system.

FIGS. 13A and 13B are schematic illustrations of Tri-ICP (FIG. 13A) andRF-ICP desalination experiment (FIG. 13B).

FIG. 14 is a schematic of simulation models of (a) RF-ICP and (b)Tri-ICP.

FIG. 15 is a schematic of an experimental configuration to evaluatepower consumption and water cost variation.

FIG. 16 shows flow velocity variation under an application of constantcurrent flux. Various flow velocities (1.2, 1.8, and 2.4 mm/s) and feedsalinities (10, 35, and 70 g/L) are applied for RF-ICP system. (N=3,error bars indicate standard deviations).

FIGS. 17A, 17B, and 17C show the salinity variation at two channeloutlets, diluate (●) and concentrate stream (▴) outlets, and one end ofintermediate stream (▪) in the RF-ICP system with salinities of 10 (FIG.17A), 35 (FIG. 17B) and 70 g/L (FIG. 17C). (N=3, error bars indicatestandard deviations).

FIGS. 18A-18C show real-time monitoring of salinity changes for RF-ICPunder a constant current flux. Corresponding experiment conditions,current density and feed salinity, are indicated in the upper leftcorner of each graph.

FIGS. 19A-19C show the calculated resistivity based on salinityvariation with feed solutions of 10 (FIG. 19A), 35 (FIG. 19B), and 70g/L (FIG. 19C). The dashed lines indicate the resistivity of the feedsolutions. (N=3, error bars indicate standard deviations).

FIGS. 20A-20C show the power consumption by three differentchannels—i.e., diluate, intermediate, and concentrate channels. (N=3,error bars indicate standard deviations).

FIG. 21 shows the change in resistivity according to salinity at 25° C.The resistivity is calculated by the data set in [12].

FIGS. 22A, 22B, and 22C show the change in the current utilization (CU)of RF-ICP and Tri-ICP versus current flux for 10 (FIG. 22A), 35 (FIG.22B), and 70 g/L (FIG. 22C).

FIG. 23A shows power consumption according to the salt removal ratio(SRRatio) and FIG. 23B shows energy per unit ion removal (EPIR)according to salt removal rate (SRRate) for RF-ICP and Tri-ICP.

FIG. 24 shows the relationship between the salt removal ratio (SRRatio)and the salt removal rate (SRRate).

FIGS. 25A and 25B show the result of numerical analysis. FIG. 25A showsthe visualized magnitude of flow velocity and PM-flow velocity towardthe diluate channel along the porous membrane. FIG. 25B shows thevisualized cation concentration and the distribution of local currentdensity, CU, and EPIR along CEM. The values are nondimensionalized bydividing by the local value in the beginning for RF-ICP at 2 mm/s ofU_(F).

FIGS. 26A, 26B, and 26C show the power consumption at various flowvelocities (0.5˜4.0 mm/s) for a fixed SRRatio (10, 30, 50, 70, and 95%)of feed solutions with 10 (FIG. 26A), 35 (FIG. 26B) and 70 g/L (FIG.26C) of concentration. (N=4, error bars indicate standard deviations).

FIG. 27 shows the optimal water cost variation with changes in equipmentsize, lifespan, and electricity cost. The optimal water cost for Tri-ICPwas calculated using experimental results from the literature [8].

FIGS. 28A, 28B, and 28C show the water cost optimization for an RO brinetreatment scenario. FIG. 28A is a schematic illustration of the process.FIG. 28B plots the cost and recovery rate changes as a function ofSRRatio for RF-ICP desalination with a fixed waste treatment cost of$5/m³. FIG. 28C plots the water cost change as a function of the wastetreatment cost. (Numbers on the line indicate waste treatment costs,$/m³).

FIG. 29 is a schematic comparing the ICP with a bifurcated channel withnormal flow (Bi-N; left side) and the CF-ICP system (referred to as abifurcated channel with counter flow, Bi-C; right side), each having 30cm of effective membrane length. In the Bi-N system shown in the figure,the feed stream is bifurcated into a diluate stream and a concentrate onthe cathodic and anodic sides of the porous membrane, respectively. Thisarchitecture is characterized by large back diffusion and osmosisbetween cation exchange membranes (CEMs). In the CF-ICP system (Bi-C),the feed stream forms two streams, the concentrate stream, which isdirected to the outlet, and the diluate stream, which passes the lengthof the channel and then flows in a direction counter to that of the feedstream.

FIG. 30 shows the concentration profiles of the Bi-C and Bi-N systems atfeed flow velocities of 0.5 mm/s and 1.5 mm/s. The graph on the rightside shows the salinity variation at the concentrate and diluate outletswith a feed salinity (C₀) of 70 g/l.

FIG. 31 shows the current utilization (CU) of the Bi-C and Bi-N systemsat feed flow velocities of 0.5 mm/s and 1.5 mm/s. The graph on the rightside shows the change in CU of Bi-C and Bi-N versus the current flux for70 g/l feed salinity.

FIG. 32 shows the concentration difference of the Bi-C and Bi-N systemsat feed flow velocities of 0.5 mm/s and 1.5 mm/s. The graph on the rightside shows the concentration (g/L) versus the current flux for 70 g/lfeed salinity.

FIG. 33 shows the power consumption for the Bi-C and Bi-N systems atfeed flow velocities of 0.5 mm/s and 1.5 mm/s. The graph on the rightside shows the power consumption according to salt removal ratio(SRRatio) for Bi-C, Bi-N, and RF-ICP for feed salinities of 70 and 100g/l.

DETAILED DESCRIPTION

A description of embodiments of the invention follows.

As used herein, the words “a” and “an” are meant to include one or moreunless otherwise specified.

Conventional electrodialysis (ED) generally operates at 80% of alimiting current regime for energy efficiency. To reduce capital costcaused by expensive membrane cost in an electrodialysis plant, theapplication of current should be increased; and this current isindispensable to enter an over-limiting current regime. However, theover-limiting current results in many side effects, such as chaoticelectroconvection in a dilute channel, and a back diffusion fromconcentrate to the dilute channel, or water splitting. The abovephenomena have a negative impact on both desalination for purificationof water and concentration for salt production.

Conventional electrodialysis (ED) for desalination facilitates saltremoval using a bipolar ion conduction employing alternating two ionexchange membranes—an anion exchange membrane (AEM) and a cationexchange membrane (CEM) (FIG. 1A) [1]. The ion depletion layers, whichdevelop next to AEM and CEM, are formed by different ion transports inthe solution and the ion exchange membrane [2,3]. It is important tonote that the development of the ion depletion layer, a desired outcomefor desalination, also largely determines the energy consumption,because of the increased electrical resistance of the ion-depletedboundary layer [4,5]. Since the thickness of the ion depletion layer islargely determined by the current (or salt removal ratio), this poses afundamental trade-off for any electromembrane desalination processes;the higher the salt removal ratio of the process becomes (i.e., thickerion depletion layer), the more resistant the cell becomes, resulting inpoor energy efficiency.

It is, however, still important to increase energy efficiency byreducing the ion depletion layer for both ED and ICP desalination. InED, the ion depletion layer has been mechanically controlled by meshspacers as a turbulence promoter or corrugated membrane for influencingthe flow profile [6,7,8]. In other words, the studies have focused oneliminating ion depletion layer by mixing the entire channel as much aspossible. Yet, the entire diluate stream inevitably becomes of highelectrical resistance (no matter how well it becomes mixed) when ED isoperated with a high salt removal ratio, resulting in high powerconsumption. Previously, a new desalination technique was proposed, socalled ion concentration polarization (ICP) desalination, using unipolarion conduction by employing only a CEM to enhance energy efficiencyusing the higher diffusivity of chloride ion (FIG. 1B) [2,3]. Thebifurcate ICP (Bi-ICP) desalination model facilitates collection ofdepleted and enriched streams via two separated channel ends. In ICPdesalination, both ion depletion and enrichment appear in the samechannel, resulting in a few unique advantages, such as co-removal oftotal suspended solids (TSS) [2,3]; however, the mixture of theion-depleted and concentrated streams should be prevented. Therefore,this represents a fundamental trade-off for any electromembranedesalination process, limiting performance and cost-effectiveness,especially when compared with reverse osmosis (RO).

A trifurcate ICP (Tri-ICP) desalination architecture was developed tofacilitate collection of thin depleted and concentrated streams, whichdevelop adjacent to a CEM (FIGS. 2A and 2B) [4,5]. In this architecture,a thin ion depletion region is extracted to achieve both good energyefficiency (only a thin depletion region is generated) and a good saltremoval ratio (incremental depleted/desalted stream is extracted). Usingthis architecture, we have demonstrated cost-effective partialdesalination of brine (from 70 g/L to 35 g/L) in a lab-scaleexperimental system [9]. However, Tri-ICP essentially relies on lowercurrent electromembrane operation and, therefore, requires recirculationof the main fluid intake. Another idea to address this tradeoff is toincorporate microporous ion-selective membranes, allowing the diluatestream to ‘penetrate’ into the membranes through large pores (100 μm),effectively eliminating the ion-depleted boundary layer and leading tohigher energy efficiency. However, this requires a rather complex systemand fluid handling architecture [10].

The present application encompasses a newly designed channel forelectrical water desalination/concentration technology. The systems andmethods described herein include:

-   -   (1) a return flow system, which results in a suppressive flow        barrier for stable ICP desalination;    -   (2) a return flow system, which results in a preventive flow        barrier for stable ICP concentration;    -   (3) a return flow system resulting in united flow barriers for        stable ICP desalination/concentration; and    -   (4) a double return flow system for conventional        electrodialysis.

ICP desalination, bifurcate, and trifurcate ICP desalination systemshave been described, for example, in U.S. Pat. App. Pub. No.2014/0374274 A1, U.S. Pat. No. 9,845,252, U.S. Pat. App. Pub. No.2017/0066665 A1, U.S. Pat. No. 9,850,146, U.S. Pat. App. Pub. No.2016/0115045 A1, Kim, et al. (2016), Scientific Reports 6:31850; andKwak, et al. (2016), Sci Rep. 6: 25349, the contents of each of whichare expressly incorporated by reference herein.

As shown in FIG. 1B, ICP desalination results in both dilute andconcentrate streams that are separately acquired between two identicalion exchange membranes (IEMs) (FIG. 1B) whereas a conventionalelectrodialysis (ED) needs alternating differently charged IEMs, e.g.,alternating an anion exchange membrane (AEM) and a cation exchangemembrane (CEM) (FIG. 1A). A trifurcate ICP desalination system andmethod is shown in FIGS. 2A and 2B, which enables the collection of thinion-depleted and ion-enriched layers, which develop next to the IEMsurface, by dividing outlets of the target stream within one channelunit (FIGS. 2A and 2B). In ICP desalination/purification, colloidalpollutant particles and charged bio-agents can simultaneously be removedfrom brackish and/or contaminated water by a nonlinear ion concentrationpolarization (ICP) phenomenon between two identical ion exchangemembranes (IEMs). The consequence of the configuration is that onlypositive (or negative) ions, but not both, participate in theconduction. Ion exchange membranes (IEMs) act as an ion filter byallowing only cations or anions to pass through. This selective iontransport initiates a unique phenomenon called ion concentrationpolarization (ICP) near the membranes, which is characterized bysignificant, dynamic perturbation in ion concentrations (also known asion depletion and ion enrichment) [11]. In 2010, S. J. Kim et al.demonstrated a microfluidic desalination device by using ICP. Twodisadvantages of this technology include chemical reactions and pHchanges near electrodes and difficulties in scale-up [12].

As discussed above, ICP desalination/concentration utilizes ICP betweentwo identical IEMs. Between two juxtaposed similar ion exchangemembranes (AEMs or CEMs), an ion depletion zone (d_(de)) and ionenrichment zone (d_(en)) are generated under an electric field. Ascations are selectively transferred through the CEMs, for example,anions are relocated in order to achieve electro-neutrality, resultingin the concentration drop (increase) in the ion depletion (enrichment)zone. The concentration drop (or salt removal) is low and spatiallygradual at relatively low voltage or current (e.g., Ohmic regime).However, at higher voltage or current (e.g., in the over-limitingregime), strong electroconvective vortex or vortices accelerate cationtransport through CEMs, allowing “relocation” of most salt ions. Theflat depletion zone occurs with significantly low ion concentration anda corresponding strong electric field in the zone, and any chargedagents (e.g., proteins and bacteria) cannot penetrate this flat zone.This ICP desalination/purification also occurs with two anion exchangemembranes (AEMs) by relocating cations, but the locations ofdesalted/brine flows are reversed.

As described above, the over-limiting current is accompanied by twophenomena, chaotic electroconvection in the dilute stream andpropagation of enriched salt from the concentrate stream, with anincrease in energy consumption. The electroconvection in the dilutestream generates a chaotic flow motion which increases energydissipation and causes an undesirable flow mixing. Highly enriched saltin the concentrate stream propagates to the dilute stream, causing adecline of salt removal efficiency. In previous configurations, thedilute and concentrate streams were on the same channel component, andthey can affect each other without any restrictions. The apparatus andmethods can improve energy efficiency for desalination and/or saltproduction by minimizing chaotic electroconvection at the dilute streamand/or blocking enriched salt propagation from the concentrate stream.Specifically, the present discussion is directed to an ICPdesalination/concentration system and method comprising a return flowsystem.

The methods described herein produce at least two streams: a stream thathas reduced ionic species and a stream with concentrated ionic species.The stream that has reduced ionic species can be referred to as the“dilute stream,” the “purified water stream,” the “diluate stream,” orthe “diluate,” interchangeably herein unless otherwise indicated. Thestream that has concentrated ionic species can be referred to as the“concentrate stream,” the “concentrated ion aqueous stream,” or the“concentrate” interchangeably herein unless otherwise indicated.

FIGS. 3 to 5 describe specific embodiments of the return flow system forICP. Specifically, FIGS. 3, 4, and 5 are schematics showing a returnflow system that results in a suppressive flow barrier, a return flowsystem that results in a preventive flow barrier, and a return flowsystem for united barriers, respectively.

FIG. 3 represents a desalination application (CF-ICP) using a returnflow system that results in a suppressive flow barrier. The inlet of thefeed stream and the outlet of the dilute stream are installed in thesame direction or on the same end of the channel (here, right side), butare separated by a porous membrane except the end of the channel for areturn flow. The inlet of the feed stream is installed at the anodicside, and the outlet of the dilute stream is installed on the oppositeside of the channel. The other stream, the discharge stream (theconcentrate stream), is discharged through the outlet on the left side(the end of the channel opposite to the inlet end). The feed streamadjacent to the porous membrane partially flows through the membrane tosuppress an electroconvection in the dilute stream. Then the feed streamat the end of the porous membrane splits and returns to the right sideof the channel to wash out the desalted mass near IEM.

The configuration shown in FIG. 3 is also referred to herein as“counter-flow” ICP (CF-ICP) for desalination application. In ED and ICP,a high salt removal ratio requires a large concentration polarization atthe membrane interface. The large concentration polarization results ina larger trans-membrane concentration difference, leading to strongerdiffusion and osmosis. This results in an increase in the total currentapplication to compensate for the reverse transfer of salt. To solvethese problems, CF-ICP has two features to address the key challenge ofenabling energy-efficient, high-current desalination. First, the diluatestream effectively passes the channel twice, increasing feed water dwelltime, but the concentrate stream flows out without any dwell timeincrease. Second, one can reduce the concentrate difference along CEM(FIGS. 8A and 8B, right side). The minimized trans-membraneconcentration difference results in reduced diffusion and osmosis.

In the CF-ICP system shown in FIG. 8A, the ion exchange membranes areCEMs, the first outlet is located on the cathodic side of the firstporous membrane, the inlet is located on the anodic side of the firstporous membrane, and the purified water stream is directed to the firstoutlet. In this CF-ICP system, the second outlet (to which theconcentrate stream is directed) is located at the return flow end. Asdiscussed above, the diluate stream passes through the channel twice,whereas the concentrate stream flows out of the second outlet (at thereturn flow end) and effectively only passes through the channel once.The return flow end is closed on the cathodic side (but open at theanodic side). In this way, the concentrate stream is directed to thefirst outlet, and the purified water stream is directed to the firstoutlet.

FIG. 4 represents a concentration application using the CF-ICP systemthat results in a preventive flow barrier. The channel configuration forthe concentration application is identical to the channel configurationfor the desalination application of FIG. 3 , except with respect to theinlet and outlet positions. The inlet of the feed stream is installed atthe cathodic side of the channel, while the outlet of the concentratestream is installed opposite direction. The feed stream flowing adjacentto the porous membrane flows through the membrane to prevent or suppressthe propagation of concentrated mass from the concentrate stream.

FIG. 5 represents a desalination and concentration application using thereturn flow system resulting in united flow barriers. The inlet of thefeed stream is located between the outlets for the dilute andconcentrate streams, and the left side of the channel is entirelyblocked. The outlet of the concentrate stream is placed on the anodicside of the channel and that of the dilute stream is placed in theopposite direction. Each outlet is separated from the inlet by a porousmembrane, except the end of the channel for the return flow. Thus, thisconfiguration includes two porous membranes. The feed stream adjacent tothe porous membranes flows through the membranes in two directions, asuppressive flow barrier for the dilute stream and a preventive flowbarrier for the concentrate stream. The feed stream splits into twodirections at the end of the porous membranes to wash out the enrichedmass and desalted mass on the IEM surface, respectively.

The configuration shown in FIG. 5 is also referred to herein as returnflow ICP (RF-ICP). It is known that an increased flow rate in the ED (ata given operating current) results in a reduction in the ion depletionregion and, therefore, in increased energy efficiency. Yet, this meansthat the same amount of salt removed (same current) for a larger volumeof water processed results in a reduced salt removal ratio for productwater. As discussed above, incorporating a return flow system re-routesthe feedwater inside the ED or ICP channels. The ‘return-flow’architecture (RF-ICP) is shown in FIGS. 7A and 7B. This system has threeunique features to address the key challenge of enablingenergy-efficient, high-current desalination. First, the fluideffectively passes the channel twice (by return flow), effectivelyincreasing the length of the channel (feedwater dwell time). Second, onecan significantly increase the output desalted flow speed compared withthe incoming flow speed, resulting in sequestration of the ion depletionregion even at high salt removal ratios. With the change in fluidbehavior, the current and deletion layer are re-distributed along themembrane (FIGS. 7A and 7B, right side). The current is more uniformlydistributed along the membrane, and the depletion layer was developedwith a flat thickness, resulting in an overall resistance reduction.

In the RF-ICP system shown in FIGS. 7A and 7B, the ion exchangemembranes are CEMs; the second outlet is located on the inlet end of thechannel; the inlet is located between the first outlet and the secondoutlet; the inlet and the second outlet are separated by a second porousmembrane that traverses the length of the channel between the ionexchange membranes and terminates at the return flow zone; and thereturn flow end is fully closed.

The flow rate of the outlet can be controlled independently. Forcollection of highly enriched mass from the concentrate stream, theoutlet flow rate of the concentrate stream can decrease. In the samemanner, the outlet flow rate of the dilute stream can increase forcollection of a large volume of desalted mass. The pore size of theporous membrane can be varied for control of suppressive and preventiveflow through the membrane.

As described above, systems, devices, and methods provide for purifyingand/or concentrating a first water stream containing ionic impuritiescomprising the steps of:

-   -   a. directing the water stream into an inlet of a channel forming        a feed stream, wherein the channel is defined, at least in part,        by a first ion exchange membrane and a second ion exchange        membrane, wherein the ion exchange membranes are juxtaposed and        characterized by the same charge, wherein the channel is further        characterized as having an inlet end and a return flow end,        wherein the inlet end is the end of the channel at which the        inlet is located, and the return flow end is the end of the        channel downstream, with respect to the inlet, from the inlet        end, the channel further comprising a first outlet and a second        outlet, wherein the inlet and at least the first outlet are        located on the inlet end of the channel and are separated by a        first porous membrane that traverses the length of the channel        between the ion exchange membranes and terminates at the return        flow zone, wherein the return flow zone is a section of the        channel at the return flow end, and wherein the return flow end        is at least partially closed;    -   b. applying an electric field across the channel causing        formation of an ion depletion zone comprising a purified water        stream and formation of an ion enrichment zone comprising a        concentrated ion aqueous stream, wherein at least part of the        feed stream enters the return flow zone and forms a first return        flow stream that flows to the opposing side of the first porous        membrane, the first return flow stream flows in the direction of        the first outlet, and at least part of the feed stream adjacent        to the first porous membrane flows through the first porous        membrane joining the return flow stream, wherein the purified        water stream is the stream directed to the first or the second        outlet, and the concentrated ion aqueous stream is the stream        directed to the other of the first and the second outlet; and    -   c. collecting the purified water stream and/or the concentrated        ion aqueous stream from the first and/or second outlet.

The ion exchange membranes can be cation exchange membranes (CEMs) oranion exchange membranes (AEMs). The electric field can be created by anelectrode and a ground, each located external and parallel to thechannel. The two ion exchange membranes can be the same or different.Strong anion or cation exchange membranes, as those products aregenerally sold in the art, are preferred. FUMASEP® FTAM-E and FTCM-E(FuMA-Tech CmbH, Germany) are suitable membranes. A suitable membrane isalso a NAFION® membrane, for example, a NAFION® perfluorinated membraneavailable, for example, from Sigma Aldrich, USA. However, others canalso be used. In particular, the term “ion exchange membrane” isintended to include not only porous, microporous, and/or nanoporousfilms and membranes, but also resins or materials through which ions canpass. Thus, in one embodiment, an ion exchange resin can be entrapped byone or more meshes (or porous membranes) in lieu of or in addition toone or more of the ion exchange membranes. In certain aspects, the ionexchange membranes comprise micrometer-sized pores (or micro pores). Inyet additional aspects, the ion exchange membranes comprisenanometer-sized pores (or nano pores). In yet further aspects, the ionexchange membranes comprise micro pores and nano pores. An exemplary ionexchange membrane comprising micro pores and nano pores has beendescribed, for example, in Kwon, et al., (2015), “A Water Permeable IonExchange Membrane for Desalination”, 19^(th) International Conference onMiniaturized Systems for Chemistry and Life Sciences October 25-29,Gyeongju, Korea, available athttp://www.rsc.org/images/LOC/2015/PDFs/Papers/1202_T.302e.pdf, thecontents of which are expressly incorporated by reference herein. Theion exchange membranes can be placed into a support, such as glass,polydimethylsiloxane, or other inert material. Thus, the support canalso contribute to the formation of the channels.

FIGS. 3 to 5 show channels formed by cation exchange membranes (CEMs).Anion exchange membranes (AEMs) can also be used in thedesalination/concentration methods described herein, but the outlets forthe purified water stream and concentrated ion aqueous streams arereversed.

In certain aspects, the ion exchange membranes are CEMs, and the firstoutlet is located on the cathodic side of the first porous membrane; theinlet is located on the anodic side of the first porous membrane; andthe purified water stream is the stream directed to the first outlet;optionally, the second outlet is located at the return flow end. Theterms “anodic side” and “cathodic side” are used in reference to theside of the channel proximal to the anode and the cathode, respectively.The second outlet can be located at the return flow end, for example,the second outlet is located on the part of the return flow end that isnot closed and that is on the same side (anodic or cathodic) as theinlet (see, for example, FIGS. 3 and 8A).

In additional aspects, the ion exchange membranes are CEMs, and thefirst outlet is located on the anodic side of the porous membrane; theinlet is located on the cathodic side of the porous membrane, and theconcentrated water stream is directed to the first outlet. The secondoutlet can be located at the return flow end; for example, the secondoutlet is located on the part of the return flow end that is not closedand that is on the same side (anodic or cathodic) as the inlet.

As described above, at least part of the feed stream enters the returnflow zone and forms a return flow stream that flows to the other(opposing) side of the porous membrane (as compared to the side of theporous membrane where the feed stream flows). The part of the feedstream that forms the return flow stream can, for example, be the feedstream that enters the closed portion of the return flow zone (whereinthe closed portion of the return flow zone is that portion adjacent tothe closed part of the return flow end). The return flow stream thenflows in the direction of the first outlet (e.g., cross-current to thefeed steam entering the inlet) and is directed to the first outlet. InFIGS. 3 and 8A, the return flow stream is the dilute stream, and theclosed portion of the return flow end is on the side of the channel inwhich the diluate is formed.

In yet additional aspects, the second outlet is located on the inletend; the return flow end is fully closed; the inlet is located betweenthe first outlet and the second outlet; and the inlet and the secondoutlet are separated by a second porous membrane that traverses thelength of the channel between the ion exchange membranes except at thereturn flow zone. Where the ion exchange membranes are CEMs, the firstoutlet is located on the cathodic side of the first porous membrane (themembrane separating the inlet and the first outlet); the second outletis located on the anodic side of the second porous membrane; thepurified water stream is the stream (e.g., the first return flow stream)directed to the first outlet; and the concentrated ion aqueous stream isthe stream (e.g., the second return flow stream) directed to the secondoutlet. At least part of the feed stream enters the return flow zone(for example, the closed portion of the return flow zone) and forms asecond return flow stream that flows to the other side of the secondporous membrane as the feed stream (the opposing side) and flows in thedirection of the second outlet. In addition, at least part of the feedstream adjacent to the second porous membrane flows through the secondporous membrane joining the second return flow stream. The purifiedwater stream and the concentrated ion aqueous streams are collected fromthe first and the second outlets, respectively. In FIGS. 5 and 7 , thereare two return flow streams: the dilute stream is directed to the firstoutlet, and the concentrate is directed to the second outlet.

FIGS. 3 to 5 show channels formed by cation exchange membranes (CEMs).Anion exchange membranes (AEMs) can also be used in thedesalination/concentration methods described herein, but the outlets forthe purified water stream and concentrated ion aqueous streams would bereversed. For CEMs, the ion concentration is depleted at the cathodeside (ion-depleted region) and concentrated at the anode side(ion-enrichment region). For AEMs, the ion concentration is depleted atthe anodic side (ion-depleted region) and concentrated at the cathodicside (ion-enrichment region). Thus, where CEMs are used as the ionexchange membranes that form the channel that provides a suppressiveflow barrier (e.g., FIG. 3 ), the inlet is located on the anodic side,and the outlet for the purified water stream is located on the cathodicside of the channel. If AEMs are used to provide a suppressive flowbarrier (e.g., in a system analogous to FIG. 3 ), then the inlet islocated on the cathodic side of the channel, and the outlet for thepurified water stream is located on the anodic side of the channel.Similarly, if AEMs are used to provide a preventive flow barrier (e.g.,analogous to the system of FIG. 4 ), then the inlet is located on theanodic side of the channel, and the outlet for the concentrated solutionis located on the cathodic side of the channel. In another example, ifAEMs are used for the united flow barrier system (e.g., analogous to thesystem of FIG. 5 ), then the outlet for the purified stream is locatedon the anodic side, and the outlet for the concentrate ion aqueousstream is located on the cathodic side of the channel.

The channels described herein include at least two outlets; for example,one outlet is for the purified water stream, and the other outlet is forthe concentrate ion aqueous stream. As explained herein, whether thepurified water stream is the stream directed to the first outlet (thereturn flow stream) or the stream directed to the second outlet dependson where the outlet is located on the end of the channel relative to theinlet (e.g., on the anodic or cathodic side), and whether the ionexchange membranes are CEMs or AEMs. The concentrate ion aqueous streamis directed to the other outlet of the at least two outlets (in otherwords, to the outlet to which the purified water stream is notdirected). As described herein, the “first outlet” is the outlet locatednext to the inlet and is thus on the same end of the channel as theinlet. In some aspects, the second outlet is located at the return flowend, which is the end of the channel opposite to the end where the inletis located. In yet other aspects, the second outlet is located on thesame end of the channel as the inlet and the first outlet. The inletcan, for example, be located between the first and second outlets.

At least one porous membrane is used to separate the inlet and the firstoutlet, and the porous membrane traverses the length of the channelexcept at the return flow end, thus the porous membrane traverses thelength of the channel and terminates at the return flow zone. Inaddition, where the second outlet is also located next to the inlet, aporous membrane is used to separate the inlet and the second outlet, andthe porous membrane traverses the length of the channel except at thereturn flow end. The porous membrane can, for example, be a non-ionicporous membrane. In addition, the porous membrane can be microporousand/or nanoporous. In certain aspects, the porous membranes comprise,for example, pores about 1 nm to about 2 μm or about 100 nm to about 2μm, or about 1 μm to about 2 μm in size. In certain additional aspects,the pores are about 1-μm diameter pores. The porous membrane(s) can belocated parallel or substantially parallel to the ion exchangemembranes. A non-ionic porous membrane is a porous membrane that is notan ion exchange membrane or that is not charged and thus does not onlyallow cations or anions to pass through. The non-ionic porous membranecan allow fluid and ions (cations and anions) to pass through.

The feed stream directed into the channel via the inlet flows from theinlet in the direction of the return flow end. At least part of the feedstream adjacent to the porous membrane flows through the porous membraneto the opposing side of the membrane (flows to the other side of themembrane as that of the feed stream entering through the inlet). Thereturn flow zone is the section of the channel at the return flow endthat is not traversed by the porous membrane and/or where the porousmembrane is not present. At least part of the feed stream that flows tothe return flow zone forms a return flow stream that flows to the otherside (or opposing) of the first porous membrane (flowing around the endof the porous membrane) and flows in the direction of the first outlet(cross current to the flow of the flow stream entering the inlet).Depending on the specific features of the channel (e.g., the location ofthe first outlet, the IEMs used), the purified water stream is thestream directed to the first or the second outlet, and the concentratedion aqueous stream is the stream directed to the other of the first andthe second outlet. The return flow stream is the purified water streamor the concentrated ion aqueous stream (depending on which stream isdirected to the first outlet).

In general, the channel formed by the two juxtaposed ion exchangemembranes does not contain a membrane carrying a charge counter to thetwo juxtaposed ion exchange membranes. The consequence of theconfiguration is that only positive (or negative) ions, but not both,participate in conduction. In other words, the ions in the electrolytesolution or aqueous stream to be purified that participate in theconduction in the apparatus, or cell, carry a common charge, while thecounterions or ions carrying the opposite charge, while present, do notparticipate in conduction. Thus, the systems and methods can exclude theuse of an apparatus that traditionally functions via electrodialysis.

The electric field can be generated by an electrode and a ground, eachlocated external and parallel to the channel. The electric field can begenerated, for example, by an anode and a cathode. An electrode can formanother channel (e.g., a second channel) with the first ion exchangemembrane; for example, an anode can form a second channel with the firstion exchange membrane. The ground or, for example, the cathode, can formyet another channel (e.g., a third channel) with the second ion exchangemembrane. The second and third channels can be filled with anelectrolyte solution. In certain aspects, the electrolyte solution isthe first water stream.

In certain additional embodiments, the first water stream comprises asalt. In yet additional aspects, the first water stream comprisesbiomolecules. The first water stream can, for example, be water with arange of salinities, for example, brackish water, seawater, producedwater, and brine. The terms “brackish water,” “produced water,” and“brine” are terms known to those of skill in the art. In certainaspects, brackish water can refer to water having a salinity less thanabout 10,000 ppm and/or having an NaCl concentration greater than about0.5M NaCl. In certain aspects, produced water can have a salinitygreater than about 30,000 ppm. In certain aspects, brine can refer towater with higher salinity than 35,000 mg/L TDS and/or water having anNaCl concentration greater than about 1M NaCl. In certain aspects, thefirst water stream can be wastewater, for example, brackish groundwater,household water rich in bacteria or other biological contaminants, orsimply murky water from various suspended solids and/or industrial heavymetal contaminants. Biomolecules include cells (such as bacteria oranimal), cellular fragments, particles (including viral particles),proteins, and nucleic acid molecules, for example.

As described herein, in some aspects, the fluid flow of the device usingcurrent-voltage responses categorized as Ohmic (1-2 V), limiting(2-2.5V) and over-limiting (>2.5 V) regimes. The electric fieldpreferably creates a boundary layer comprising at least oneelectroconvective vortex proximal to at least one of the two juxtaposedion exchange membranes. The electric field is created by an electrodeand a ground, each located external and parallel to the channel. Ingeneral, the electrode forms a second channel with the first of said twojuxtaposed ion exchange membranes, and the ground forms a third channelwith the second of the two juxtaposed ion exchange membranes. Thesechannels are generally filled with an electrolyte solution, which canconveniently be the water stream to be purified or concentrated.

This disclosure further encompasses a device comprising the channel andthe return flow system, as described herein.

This disclosure also encompasses an electrodialysis system and methodincluding a return-flow system. Electrodialysis is anelectrically-driven membrane desalination technology that removes anionsthrough an anion exchange membrane (AEM) and a cation through a cationexchange membrane (CEM). Specifically, the systems and methods include adouble flow barrier using a return-flow system for EDdesalination/concentration. FIG. 6 is an example of such a system.

FIG. 6 is a schematic showing a return flow system facilitatingformation of a double flow barrier for a conventional electrodialysis.The installation and operating mechanism are similar to the united flowbarrier system for ICP desalination/concentration shown in FIG. 5 , butthe purpose of the barrier is different. In conventionalelectrodialysis, the dilute and the concentrate channel are separated byan IEM, whereas they are placed in the same channel in the ICPdesalination/concentration. For this reason, the united flow barriersthat have two different barrier functions, a suppressor and a preventer,are operated as double flow barriers that have two barriers in thechannel, but the same feature, in the conventional electrodialysis. Thereturn flow system and barriers result in the collection of a thindepleted and enriched mass layer next to the IEM surface and can resultin a reduction of energy consumption.

In certain aspects, a system or method of purifying and/or concentratinga first water stream containing ionic impurities by electrodialysiscomprises the steps of:

-   -   a. directing the water stream into an inlet of a first channel        and into an inlet of a second channel of an electrodialysis        unit, forming a first feed stream and a second feed stream,        respectively,        -   wherein the electrodialysis unit comprises at least three            stacked ion exchange membranes (IEMs), wherein the first and            the third IEMs have the same charge polarity, and the second            IEM has the opposite charge polarity, and further wherein            the second IEM is arranged between the first and the third            IEMs,        -   wherein the first channel is defined, at least in part, by            the first and the second IEMs, wherein the second channel is            defined, at least in part, by the second and third IEMs, and            wherein the first channel is on the anodic side of the unit,            and the second channel is on the cathodic side of the unit,        -   wherein the first channel and the second channel are each            further characterized as having an inlet end and a return            flow end, wherein the inlet end is the end of the channel at            which the first inlet and the second inlets, respectively,            are located, and the return flow end is the end of the            channel downstream from the inlet end;        -   the first channel and the second channel each further            comprise two outlets on the inlet end of the channels,            wherein the first inlet is located between the two outlets            of the first channel, and the second inlet is located            between the outlets of the second channel,        -   wherein the first inlet is separated from the two outlets of            the first channel by two porous membranes, respectively,            that traverse the length of the first channel between the            first and second IEMs, and terminate at the return flow zone            of the first channel, wherein the return flow zone is a            section of the channel at the return flow end, and wherein            each return flow end is fully closed,        -   wherein second inlet is separated from the two outlets of            the second channel by two porous membranes, respectively,            that traverse the length of the second channel between the            second and third IEMs; and terminate at the return flow zone            of the second channel, wherein the return flow zone is a            section of the channel at the return flow end, and wherein            each return flow end is fully closed;    -   b. applying an electric field across the first and the second        channels, wherein the electric field causes formation of two        purified water streams in the first or the second channel and        formation of two concentrated water streams in the other of the        first and the second channels;        -   at least part of the first feed stream enters the return            flow zone of the first channel and forms two return flow            streams that flow to the opposing side of the porous            membranes and the return flow streams flow in the direction            of the outlets, and at least part of the first feed stream            adjacent to the porous membranes flows through the membranes            joining the return flow streams,        -   at least part of the second feed stream enters the return            flow zone of the second channel and forms two return flow            streams that flow to the opposing side of the porous            membranes and the return flow streams flow in the direction            of the outlets, and at least part of the second feed stream            adjacent to the porous membranes flows through the membranes            joining the return flow streams, and        -   wherein the purified water stream is the stream directed to            the outlets of the first or the second channel, and the            concentrated ion aqueous stream is the stream directed to            the outlets of the other of the first and the second            channel; and    -   c. collecting the purified water stream and/or the concentrated        ion aqueous stream from the outlets of the first and/or second        channels.

The porous membranes can, for example, be non-ionic porous membranes.The non-ionic porous membranes can be the same or different.

In certain aspects, the first and third IEMs are cation exchangemembranes, the second IEM is an anion exchange membrane; the fluiddirected to the outlets of the first channel is the concentrated ionaqueous stream, and the fluid directed to the outlets of the secondchannel is the purified water stream.

In yet additional aspects, the first and third IEMs are anion exchangemembranes, and the second IEM is a cation exchange membrane, wherein thefluid directed to the outlets of the first channel is the purified waterstream, and the fluid directed to the outlets of the second channel isthe concentrated ion aqueous stream.

The systems and methods can combine the concepts of ICP andelectrocoagulation (EC) in one step. The device comprises anelectrolytic cell with a cation exchange membrane (CEM) that separatesthe solutions in contact with anode and cathode, respectively. Throughthis CEM, only cations can be transported according to the direction ofthe electric field applied. The anode is composed of a metal, typicallyaluminum or iron, and is used to provide metal ions forelectrocoagulation. This hybrid system can remove both salt andparticles. It has the flexibility to treat various types of wastewater,whether it be brackish groundwater, household water rich in bacteria orother biological contaminants, or simply murky water from varioussuspended solids and/or industrial heavy metal contaminants. The hybriddevice can also be used for produced water treatment, such as is foundin the shale gas industry, where suspended solids removal is crucial forre-use in hydraulic fracturing. The product from the CEM outlet is freeof salt and suspended solids. This water can be used as a fresh feedsolution, which is mixed with the produced water, for hydraulicfracturing. Typically, 90% of produced water is recycled in shale gaswells, mixed with fresh feed. Therefore, a relatively small portion ofdesalinated water may be sufficient to be used as fresh feed in producedwater recycling.

Similarly, the ICP-EC system has a wide range of applications inwastewater treatment systems. It can be used as a single-device watertreatment system. It does not require additional pre-treatment. It willbe mainly used for the removal of small particles, including suspendedsolids, oil droplets, organic chemicals, and biological organisms, witha small desalination capacity in addition, since the system is mainlydriven by electricity and does not require a high-pressure system, itcan be made in small scale and is suitable for a localizedwater-treatment system. By having shared electrodes for the twoindependent processes, the single electrochemical system can achieveboth pre-treatment and desalination. The system and method make use ofthe unavoidable Faradaic reaction near the anode to remove nonionicparticles, reducing the unnecessary voltage drop and thus the energyconsumption.

The salt removal ratio is a parameter to indicate the desalting abilityof devices. By measuring the concentration (or conductivity) of sampleflows C₀ and that of the desalted flow C_(desalted), we can figure outhow many salt ions are removed from the discrepancy between the twoconductivities. The salt removal ratio is a non-dimensional form of theamount of desalted ions by the initial ion concentration (orconductivity):

$\begin{matrix}{{{Salt}{removal}{ratio}} = \frac{C_{0} - C_{desalted}}{C_{0}}} & (1.1)\end{matrix}$

The concentrations can be converted from the measured conductivity, a,in experiments with given molar conductivities of electrolytes. Here, weuse only dilute binary electrolytes (z⁺=z⁻=1), 10 mM KCl, NaCl, and LiClsolutions. Then, the equation for conversion is:

$\begin{matrix}{{{C_{i}\left\lbrack {{{mol}/m^{3}} = {mM}} \right\rbrack} = {\frac{\sigma}{\Lambda_{+ {,i}} + \Lambda_{- {,i}}}\left\lbrack \frac{S/m}{{S \cdot m^{2}}/{mol}} \right\rbrack}},} & (1.2)\end{matrix}$

where Λ_(+,i) and Λ_(−,i) are the molar conductivity of cation andanion. The molar conductivities of Cl⁻, K⁺, Na⁺, and Li⁺ are 7.63, 7.36,5.01, and 3.87 [10³ S m² mol⁻¹], respectively, which are connectedclosely with their diffusivity.

To compare different desalination devices, energy consumption isfrequently measured. In electrochemical desalination systems, energyconsumption for desalination is electrical power consumption(multiplication of current, I, and voltage, V) divided by the flow rateof desalted water, Q_(desalted), per one cell:

$\begin{matrix}{{{Energy}{consumption}} = {{\frac{IV}{Q_{desalted}/N}\left\lbrack {{Wh}/L} \right\rbrack}.}} & (1.3)\end{matrix}$

While energy consumption is an important metric determining the economicviability of the desalination technique, it cannot represent thedesalination energy efficiency of the system. We, therefore, considerenergy consumption to remove a single ion—i.e., energy per ion removal,which can be obtained by dividing energy consumption by the amount ofremoved ions and non-dimensionalizing by thermal energy, k_(B)T (=2.479kJ/mol):

$\begin{matrix}{{{Eneregy}{per}{ion}{removal}} = {\frac{{NIV}/Q_{desalted}}{k_{B}{T\left( {C_{0} - C_{desalted}} \right)}}.}} & (1.4)\end{matrix}$

Energy per ion removal (EPIR) is a parameter representing howefficiently energy is consumed to reject ions by combining the conceptof energy consumption and salt removal ratio. However, it is noted thatthe salt removal ratio or the value of conductivity drop can be checkedtogether, because better energy per ion removal does not necessarilyrepresent better desalting performance.

Current efficiency describes the ratio of rejected ions in desalted flowand ions transferred at the electrodes. The following equation ismodified to obtain current efficiency from the concentration differencesof initial sample flow and desalted flow:

$\begin{matrix}{{{current}{efficiency}} = {\frac{{zFQ}_{desalted}\left( {C_{0} - C_{desalted}} \right)}{NI}.}} & (1.5)\end{matrix}$

Last, area efficiency represents the amount of desalted ions per unitarea of the working membranes or electrodes:

$\begin{matrix}{{{{Area}{efficiency}} = {\frac{C_{0} - C_{desalted}}{A}\left\lbrack {{mM}/m^{2}} \right\rbrack}},} & (1.6)\end{matrix}$

where A is the working area of the IEMs here. The most significant costof an electrochemical desalination system is the membrane cost;therefore, higher area efficiency would be economically favorable.However, there is usually a trade-off between area efficiency and energyconsumption; if one increases area efficiency to enhance the saltremoval ratio with a limited-size device by applying higher electricpotential, energy consumption will increase. If one uses a larger systemfor better salt rejection at a fixed voltage or current, area efficiencybecomes lower.

The platforms described here, ICP with two CEMs (2CEM), or AEMs (2AEM),and ED, are fabricated to study the differences and any potentialadvantages of each technique. The height h, width w, and length L of theworking channel is 0.2, 2, and 10 mm, respectively. The area of workingIEMs is, therefore, 2×10⁻⁶ m². Three different electrolytes (10 mM KCl,NaCl, and LiCl) with 10 mM concentration are used to observe the effectof asymmetric molar conductivity (or diffusivity) of cation and anion.The flow rate between IEMs is 20 μL/min, so the desalted flow rates,Q_(desalted), are 10 μL/min for the ICP platform and 20 μL/min for ED.The electrodes are rinsed with the same electrolytes (KCl or NaCl orLiCl) with 30 μL/min; dibasic buffer solution is not used here to supplythe same cations or anions within the sample water.

Current responses on applied voltage from 0 to 10 V have been measuredto overview the ICP and ED systems' characteristics. As described inU.S. Pat. No. 9,850,146, which is incorporated herein by reference, thetransition from Ohmic to over-limiting regimes are observed with theslope changes near 2V. Interestingly, the current-voltage curves of EDand the ICP platform with two CEMs are almost the same, but the curvesof the ICP platform with two AEMs are located above even with the sameelectrolytes. This indicates two major characteristics of the ICP and EDplatforms; the current responses are governed i) by the conducting ions(cations in 2CEM and anions in 2AEM) or ii) by the slower ions (cationsin ED). The movement of Cl⁻ governs the ICP platform with two AEMs withKCl, NaCl, and LiCl solutions. The movement of cations governs the ICPplatform with two CEMs and ED, because chorine ion has a higher molarconductivity than cations here. If we place ions in the order of highermolar conductivity (proportional to electrophoretic mobility ordiffusivity), it is Cl⁻>K⁺>Na⁺>Li⁺. Accordingly, in ICP with 2CEM andED, the current values with K⁺ are higher than that with Na⁺ and Li⁺.

The phenomenon by previous linear ICP analysis is that a limitingcurrent density (LCD) is linearly proportional to the diffusivity (ormolar conductivity) of conducting ions. Here, the limiting current canbe selected at the location where the current-voltage curve is bent.

For quantifying desalting performances of two types of ICP platform andED, we record voltage responses, the conductivity drop of desaltedflows, and visualized ion concentration/flow profiles with fluorescentdyes during 300 sec at a constant applied current (Ohmic regime: 5, 10μA and over-limiting regime: 20, 30, 50, 75, 100, 150, 200 μA) and agiven flow rate (20 μL/min) of various aqueous solutions with 10 mM KCl,NaCl, and LiCl. Based on the given, controlled, and measured parameters,we also obtain the salt removal ratio, energy consumption, energy perion removal, current efficiency, and area efficiency for all datapoints. As can be seen, most parameters have similar values in the Ohmicregime (5 and 10 μA) with lower current and voltage (<2 V), but thereare clear differences in the over-limiting regime. This extensivedataset of three different systems with three different electrolytesreveals many interesting trends and elucidates the differences betweenthe ICP platform and ED with nonlinear ICP.

First, the voltage-current responses show similar tendencies.Correspondingly, the energy consumptions of ED and ICP with 2CEM arematched when the same electrolyte is used. In the case of ICP with 2AEM,chlorine ions can move faster with higher molar conductivity, resultingin lower cell resistance, lower voltage responses at a given current,and lower energy consumptions than the other two systems.

However, salt removal ratio of ICP with 2AEM are worse than both ICPwith 2CEM and ED; ICP with 2CEM shows a larger salt removal ratio thanED, meaning that with the same amount of driving current, ICP (2CEM) canmove more ions from the desalted flow output. It is noted that with afaster cation (K⁺>Na⁺>Li⁺), the salt removal ratio is constant (ED) orhigher (ICP with 2CEM) or lower (ICP with 2AEM). This ambitendency ofthe salt removal ratio at a constant applied current is also shown inthe current efficiency. The current efficiency of ICP with 2CEM (2AEM)is always better (worse) than ED, and the trend is magnified as thecation molar conductivity is lower. This phenomenon will be discussed indetail in the next section.

Energy per ion removal represents the combined efficiency of both energyconsumption and salt removal. Energy per ion removal of ICP with 2AEMhas the lowest values, like energy consumption. However, the energy perion removal of ICP with 2CEM becomes better than that of ED because ofthe higher salt removal ratio of ICP with 2CEM and that of ED, even whenthe energy consumptions are the same. In all three systems, removingslow ions (Li t) requires more energy than the other faster ions (K⁺ andNa⁺). Energy per ion removal in the over-limiting regime is O (10³k_(B)T), but it becomes O(10 k_(B)T) in the Ohmic regime, which iscomparable with state-of-the-art CDI systems. While the operation in theOhmic regime (applied current <20 μA) shows better energy efficiency(i.e., energy per ion removal), the area efficiency is significantlylow. This enlightens us about the trade-off in the optimization ofdesalting processes; better energy per ion removal and worse areaefficiency (e.g., CDI or Ohmic ED), or a higher salt removal ratio andarea efficiency but worse energy per ion removal (e.g., nonlinear ED orICP). The former is ideal for achieving the maximum energy efficiencybut challenging to deal with large amounts of salts (high salinity feedwater). The latter can handle high salinity feed water (due to a highsalt removal ratio), and the system size can be minimized at the cost ofhigher energy expense per ions removed.

As discussed above, current-voltage responses in ICP and ED platformswith various salts can be largely expected from the linear and nonlinearICP model. However, the trend of the salt removal ratio is exponible fora deeper understanding of ion transport in the ICP desalination process,along with energy per ion removal and current efficiency.

The systems and methods are illustrated by the following non-limitingexamples.

EXEMPLIFICATION Example 1: Return Flow Ion Concentration PolarizationDesalination: A New Way to Enhance Electromembrane Desalination

A novel return flow (RF) electromembrane desalination process wasdeveloped where direct control of the flow path effectively limits thegrowth of the ion-depletion region, thereby resulting in both high saltremoval and high energy efficiency. FIG. 11D shows a schematicillustration of RF-ICP desalination with three channels separated by twoporous membranes. RF-ICP has the same channel architecture as Tri-ICP,which has the three channels composed of a concentrate channel on theanodic side, a diluate channel on the cathodic side, and an intermediatechannel between them. However, the intermediate channel outlet ofTri-ICP is replaced by the feed inlet of RF-ICP, and the feed inlet ofTri-ICP is entirely closed. Due to the course of flow and its pressuredistribution by the channel configuration, two flows appear as follows(FIG. 11F): firstly, the channel configuration results in a flow, whichreturns to the outlet next to the inlet, so called a return-flow(RT-flow). The feed solution enters through the inlet of theintermediate channel. Then, it flows to the end of the channel andreturns to the outlet of the two channels, separating into two channels,a diluate and a concentrate channel. The RT-flow sweeps a mass, therebycollecting the thin ion-depleted and concentrated layers, developedadjacent to CEM. Secondly, the pressure difference generates the otherflow that flows from the intermediate channel to both side channels viathe porous membranes (PM-flow). As the fluid flows along the channel,fluid pressure decreases due to energy loss due to friction. Thispressure loss makes it possible to have higher pressure distribution inthe intermediate channel than the others. It leads to an increase inpressure difference along the porous membrane. The increase in pressuredifference promotes a gradual increase in the flow rate of PM-flow,resulting in a progressive increase in the flow rate of the diluate andconcentrate stream. For the diluate stream, this phenomenon leads to anincrease in mass transfer leading to a flattened depletion boundarylayer with a uniform current distribution along CEM, while in the caseof a normal electromembrane process, the current decreases with athicker depletion boundary layer along CEM due to fixed mass transfer(FIG. 11G) [13,14]. For the concentrate stream, the presence of PM-flowreduces a diffusion flux from the concentrate stream. The diffusion fluxincreases as it gets closer to the outlet due to the increase inconcentration of the concentrate stream, but the pressure differencealso increases, further reducing the diffusion flux.

The net outcome of this architecture is the development of a relativelyeven ion-depleted region across the system, since the regions withlargest depletion region thickness (near the inlet/outlets on the left)are met by highest restricting flow from the intermediate flow. Thisdifference will result in many important advantages, both in terms ofachieving high salt removal and high energy efficiency. In this example,we demonstrate this new architecture by demonstrating the treatment ofthree concentrations of salt water (10, 35 and 70 g/L, which representbrackish water, seawater, and highly saline brine, respectively) toevaluate the technical and economic feasibility of RF-ICP desalination.

1 Materials and Methods 1.1 Device Fabrication

The modified lab-scale ICP desalination is prepared, with configuration,fabrication, and operation, as described and demonstrated in a previouswork [9]. The spacer comprised three channels—i.e., the diluate,intermediate, and concentrate channels. The intermediate channel isfabricated with clear cast acrylic sheet with a 1.6-mm thickness; andthe nanoporous membrane (poly-carbonate membrane filter with 200-nmpores, Sterlitech Co., Kent, WA, USA) were attached to both sides ofintermediate channel. Then, the diluate and concentrate channels,prepared with silicon rubbers with a 0.8-mm thickness, were attached toboth sides. Three spacers were stacked and divided by four pieces ofCMX, heterogeneous Neosepta CMX (Astom Co., Japan) with 15×5 cm² of theeffective membrane area.

1.2 System Operation and Measurement

Three sodium chloride (S5886, Sigma-Aldrich, Co., St. Louis, MO, USA)solutions with a concentration of 10, 35, 70 g/L are prepared forrepresentative salinity for brackish water seawater and highly salinebrine, respectively. Sodium sulfate (239313, Sigma-Aldrich, Co.)solution with a concentration of 0.6 M is used in the electrode rinsingchannel. In order to apply the sodium chloride solutions, thehydrodynamic pressure is generated by a peristaltic pump (Masterflex®L/S pump, Cole-Parmer Instrument Company, LLC., Vernon Hills, IL). Thesodium sulfate solution with 300 ml/m is recirculated by a circulationpump (McMaster, Robbinsville, NJ, USA). Flow rates for the diluate andconcentrate stream are controlled by a needle valve (7792K55, McMaster)and monitored by a flowmeter (4350K45, McMaster). The real-timeconductivity change is monitored by a flow-through conductivity probe(16-900 Flow-thru Conductivity Electrode, Microelectrode, Inc., Bedford,NH, USA); and then the diluate and concentrate solutions are collectedafter 5 minutes when the flow-through conductivity probe shows asaturated conductivity. The collected solutions are measured by anelectrode conductivity cell (013610MD, Thermo Fisher Scientific Inc.,Cambridge, MA, USA). The DC power supply (9205, B&K Precision Cor.,Yorba Linda, CA, USA) was used for an application of constant currentand the Digital multimeter (5491B, B&K Precision Cor.) was used for avoltage drop between spacers.

Experiment Set-Up and Analysis for the Studies on the Feasibility ofRF-ICP Desalination System:

As shown in FIG. 12 , two flow rate sensors and two needle valves areinstalled to measure and control the flow rate of the concentratechannel outlet (Q_(C,out)) and diluate channel outlet (Q_(D,out)),respectively. Normally, the outflow from the intermediate channeldirectly returns to both the concentrate and diluate channel, but theintermediate channel extended with a tube and split into two directionsto measure the flow rate of return flow (RT-flow) to the concentratechannel (Q_(C,R)) and diluate channel (Q_(D,R)). The two RT-flows arebalanced by the needle valves to have the same flow rate beforeapplication of an electric field. The flow through the porous membranes(PM-flow) towards the concentrate (Q_(C,P)) and diluate channel(Q_(D,P)) is calculated as the difference between the flow rate ofoutflow and RT-flow. The conductivities of the outlets of each channelare measured. The flow velocities of concentrate outlet (u_(C,out)),diluate outlet (u_(D,out)), RT-flow to concentrate (u_(C,R)) and re-flowto diluate (u_(D,R)) are calculated by dividing the corresponding flowrate by the width and height of the channel. The flow velocities ofPM-flow to concentrate (u_(P,C)) and diluate (u_(P,D)) are calculated bydividing the corresponding flow rate by the width and length of theporous membrane. The channel and porous membrane have the same width (35mm), but the length of the porous membrane (150 mm) is 187.5 timeslonger than the height of the channel (0.8 mm). To calculate powerconsumption within the channel, the voltage drop within the channel(V_(ch)) is measured, and the average resistivity (ρ) of each channel iscalculated based on the measured conductivity (σ) with followingequation:

$\rho = {\frac{1}{\sigma}.}$

The approximate power consumption (P) of each channel is calculatedbased on the resistivity of the channel as below:

${P_{I} = {{\frac{I^{2}}{Q_{F}} \cdot \rho_{I}}\frac{h_{I}}{wl}}},{P_{C} = {{{\frac{I^{2}}{Q_{C,{out}}} \cdot \rho_{C}}\frac{h_{C}}{wl}{and}P_{D}} = {{\frac{I^{2}}{Q_{D,{out}}} \cdot P_{D}}\frac{h_{D}}{wl}}}},$

where subscripts, I, C, and D, denote intermediate, concentration, anddiluate, respectively. I indicates current. h, w and l are the height,width, and length of the channel, respectively.

Experiment Set-Up and Analysis for Studies on the Comparison of RF-ICPand Tri-ICP Desalination:

Tri-ICP (FIG. 13A) and RF-ICP (FIG. 13B) are schematically illustratedin FIGS. 13A and 13B. Both systems have the same spacer dimension andmembrane area. The different feed flow rate is applied to have the sameflow velocity at the diluate and concentrate outlet. The three channelshave the same width and length but have different channel heights at aratio of 1:2:1 (diluate: intermediate: concentrate). RF-ICP has half ofthe feed flow rate of Tri-ICP in order to have the same flow rate ofdiluate and concentrate for both systems. This flow path improves therecovery rate from 25% to 50% without any additional flow control. Theoutlet flow rates are monitored by a flow meter to maintain a constantflow rate.

The current utilization (CU), power consumption (P), and energy per unition (EPI R) is calculated as follows:

${{CU} = \frac{{zFQ}_{{out},D}\left( {C_{F} - C_{D}} \right)}{NI}},$${P = \frac{{IV}_{eff}}{{NQ}_{{out},D}}},{and}$${{EPIR} = \frac{P}{{zk}_{b}{T\left( {C_{F} - C_{D}} \right)}}},$

where z is ion valence, F is Faraday's constant, I is current, Nis thenumber of membrane pair, C_(F) is feed ion concentration C_(D) is ionconcentration of diluate flow, k_(b) is Boltzmann constant, T istemperature, and Q_(D,out) is the diluate flow rate.

Two parameters, salt removal ratio (SRRatio) and salt removal rate(SRRate), are applied to evaluate how many ions are removed, as follows:

${{{SSRatio}(\%)} = {\frac{C_{D}}{C_{F}} \times 100}};{and}$${{SSRate}\left( {{µg}/{s \cdot {cm}^{2}}} \right)} = {\left( {C_{F} - C_{D}} \right) \cdot Q_{{out},D} \cdot {\frac{1}{A_{mem}}.}}$

where A_(mem) is the effective membrane area.

Numerical Analysis:

A numerical simulation result is obtained after solving the governingequations, including Nernst-Planck equations for ion transport (Eq.1-2), Poisson's equation for the dependence of electric potential fieldson the ion concentrations (Eq. 3-4), and Navier-Stokes and continuityequations (Eq. 5-6) for fluid motions inside the channel. Dimensionlessforms of these equations are as follows:

$\begin{matrix}{{\frac{1}{{\overset{\sim}{\lambda}}_{D}}\frac{\partial C_{\pm}}{\partial\overset{\sim}{t}}} = {- {\overset{\sim}{\nabla} \cdot {\overset{\sim}{J}}_{\pm}}}} & (1)\end{matrix}$ $\begin{matrix}{{{\overset{\sim}{J}}_{\pm} = {{- {{\overset{\sim}{D}}_{\pm}\left( {{\nabla{\overset{\sim}{C}}_{\pm}} + {Z_{\pm}{\overset{\sim}{C}}_{\pm}{\nabla\overset{\sim}{\Phi}}}} \right)}} + {{Pe}\overset{\sim}{U}{\overset{\sim}{C}}_{\pm}}}},} & (2)\end{matrix}$ $\begin{matrix}{{{{\overset{\sim}{\lambda}}_{D}^{2}{\nabla \cdot \left( {\nabla\overset{\sim}{\Phi}} \right)}} = {- {\overset{\sim}{\rho}}_{e}}},} & (3)\end{matrix}$ $\begin{matrix}{{{\overset{\sim}{\rho}}_{e} = {{Z_{+}{\overset{\sim}{C}}_{+}} + {Z_{-}{\overset{\sim}{C}}_{-}}}},} & (4)\end{matrix}$ $\begin{matrix}{{{\frac{1}{Sc}\frac{1}{{\overset{\sim}{\lambda}}_{D}}\frac{\partial{\overset{\sim}{C}}_{\pm}}{\partial\overset{\sim}{t}}} = {{- {\nabla\overset{\sim}{P}}} + {\nabla^{2}\overset{\sim}{U}} - {{{Re}\left( {\overset{\sim}{U} \cdot \nabla} \right)}\overset{\sim}{U}} - {{\overset{\sim}{\rho}}_{e}{\nabla\overset{\sim}{\Phi}}}}},{and}} & (5)\end{matrix}$ $\begin{matrix}{{{\nabla \cdot \overset{\sim}{U}} = 0},} & (6)\end{matrix}$

where {tilde over (t)}, {tilde over (C)}_(±), {tilde over (Φ)}, Ũ and{tilde over (P)} denote the dimensionless time, concentration of cations(+) and anions (−), electric potential, vector of fluid velocity, andpressure, respectively. These quantities are normalized by thecorresponding reference values of time, ionic concentration, electricpotential, velocity, and pressure, respectively as follows:

$\begin{matrix}{{{\tau_{0} = \frac{l_{0}^{2}}{D_{0}}};{C_{0} = C_{bulk}};{\Phi_{0} = \frac{k_{B}T}{Ze}};{U_{0} = \frac{\varepsilon\Phi_{0}}{\eta l_{0}}};{P_{0} = \frac{\eta U_{0}}{l_{0}}}},} & (7)\end{matrix}$

where C₀ is the concentration scale, l₀ is the characteristic lengthscale, D₀ is the average diffusivity, k_(B) is the Boltzmann constant, Tis the absolute temperature, e is the elementary charge, Z=|Z_(±)| ision valence, η is the dynamics viscosity of solution, and ε is thepermittivity of the solvent. Parameters {tilde over (D)}_(±)=D_(±)/D₀,{tilde over (λ)}_(D)=λ_(D)/l₀, and {tilde over (ρ)}=ρ_(e)/C₀ aredimensionless diffusion coefficient, the Debye length, and the spacecharge, respectively. Pe=U₀l₀/D₀, Sc=η/ρ_(m)D₀, and Re=U₀l₀ ρ_(m)/η arethe Péclet number, the Schmidt number, and the Reynolds number,respectively.

Simulations were performed using an in-house code, which solved theabove set of coupled Poisson-Nernst-Planck-Navier-Stokes equationsdirectly on a two-dimensional domain. These equations (Eq. 1-6) arediscretized using the finite volume method; the nonlinearity of theequations was treated utilizing Newton's method; and discretized linearsystems were solved using the GMRES method. The equations were solvediteratively until convergence was reached for all variables. The detailsabout the simulation method can be found in R. Kwak, V. S. Pham, K. M.Lim, and J. Y. Han, Phys Rev Lett, 2013, 110; and in V. S. Pham, Z. R.Li, K. M. Lim, J. K. White, and J. Y. Han, Phys Rev E, 2012; 86.

We consider numerical simulation models of RF-ICP and Tri-ICP sketchedin FIG. 14 . The two systems have the same channel configuration. Theheight and length of the channel are H and 5H, respectively. The lengthof the channel is enough to capture all the dynamics of the ICPdesalination phenomenon even though it does not cover the whole channellength (15 cm). The thickness of the porous wall is 0.005H, which isvery thin compared to the channel height (H). For the RF-ICP model, thefunction, ƒ(x), is applied to a porous wall having flow profiles with avalue that reaches a maximum near the inlet and decreases linearlyfurther from the inlet. The magnitude of the flow rate through theporous wall is calculated so that the total flowrate is approximately25% of the inlet flowrate. In the Tri-ICP model, a Hagen-Poiseuillepressure-driven flow of electrolyte solution is defined at the inletboundary. Since the inlet of Tri-ICP is identical to the channel height,while the RF-ICP's inlet is two times smaller, the inlet flowrate ofTri-ICP needs to be two times higher than that of RF-ICP to equalizetheir dilute flowrate, which is 25% of the inlet flowrate. Allparameters used here are described in Table 1.

TABLE 1 Parameters used in the simulation: Symbol Description Value UnitU₀ Average feed velocity 0.5 and 2.0 mm/s Φ Electric potential 10 V C₀Bulk concentration 10 mol/m³ D₊ Diffusion coefficient of cation 1.33 ×10⁻⁹ m²/s D⁻ Diffusion coefficient of anion 2.03 × 10⁻⁹ m²/s ρ_(m) Massdensity 1000 kg/m³ H Height of channel 3.175 × 10⁻³  m k_(B) Boltzmannconstant 1.381 × 10⁻²³ J/K T Absolute temperature 300 K η the dynamicsviscosity of solution  8.9 × 10⁻⁴ Pa · s λ_(D) Debye length 4.356 ×10⁻⁹  m

Experimental Set-Up and Analysis for the Power Consumption Analysis ofRF-ICP:

The experimental set-up is schematically illustrated in FIG. 15 . ThreeRF-ICP desalination spacers are stacked with the cation exchangemembranes, alternatively. The effective voltage is measured in the samemanner as in the previous measurement. Two platinum electrodes areplaced next to the membrane to measure the voltage drop in the channel.The experimental conditions are outlined in Table 2.

TABLE 2 The experimental conditions: Feed Feed Diluate Average feedsalinity Stack flow rate flow rate flow velocity (C_(F), g/L) (#)(Q_(F), ml/min) (Q_(D), ml/min) (U_(F), mm/s) 10, 3  5 2.50 0.5 35, 10 51.0 70 15 7.5 1.5 20 10 2.0 30 15 3.0 40 20 4.0

Water cost calculation:

Watercost = Capitalcost + Operatingcost,${{{Capital}{{cost}{}\left( {\$/m^{3}} \right)}} = {{\frac{{Used}{membrane}{cost}(\$)}{{Output}{Flow}{volume}{per}{{life}{}\left( m^{3} \right)}} \times {Annualized}{Factor}} = {\frac{A_{m} \times K_{Q}}{Q_{d} \times T} \cdot \frac{\left( {1 - R} \right)^{T} - 1}{T \times R}}}},{and}$${{Operating}{cost}\left( {\$/m^{3}} \right)} = {{{Power}{consumption}\left( {{kWh}/m^{3}} \right) \times {Electricity}{rate}\left( {\$/{kWh}} \right)} = {\frac{V \times I}{Q_{d}} \cdot K_{E}}}$

where A_(m), K_(Q) and Q_(d) are membrane area (m²), area normalizedequipment cost ($750/m² membrane), and diluate flow rate (m³/h),respectively. T and R are lifespan (year) and annual interest rate(10%). V, I and K_(E) are voltage (V), current (A), and electricity cost($/kWh).

Totalwatercost(TC) − ROcost(RC) + RF − ICPcost(RIC) + Wastetreatmentcost(WTC),${{TC}\left( {\$/m^{3}} \right)} = \frac{\begin{matrix}{{{{RC}\left( {\$/m^{3}} \right)} \times {Q_{{RO},p}\left( {m^{3}/h} \right)}} + {{{RIC}\left( {\$/m^{3}} \right)} \times}} \\{{Q_{{RO},b}\left( {m^{3}/h} \right)} + {{{WTC}\left( {\$/m^{3}} \right)} \times {Q_{{ICP},b}\left( {m^{3}/h} \right)}}}\end{matrix}}{Q_{{RO},p}\left( {m^{3}/h} \right)}$${{{Recovery}{rate}(\%)} = \frac{Q_{{RO},p}}{Q_{F}}},$

where Q_(RO,p) and Q_(RO,b) are the produced water and brine from RO,respectively. Q_(ICP,b) is the brine from RF-ICP.

2 Results and Discussion 2.1 Studies on the Feasibility of the RF-ICPDesalination System

We evaluate the feasibility of the RF-ICP desalination system byexamining feed flow (Q_(F)) and feed salinity (C_(F)) variation underthe application of a constant current flux (details of the experimentalset-up are given in FIG. 12 ). As shown in the figure, the flow rate ofPM-flow (Q_(P)) and RT-flow (Q_(R)) are measured at four points; and theaverage flow velocity of PM-flow (U_(P)) and RT-flow (U_(R)) arecalculated with the area of the porous membrane surface (15×3.5 cm²) andthe channel cross section (0.8×3.5 cm²). Both U_(P) and U_(R) for thediluate and concentrate streams show a symmetric flow distribution dueto the symmetric channel configuration without an application of currentflux. It is observed that U_(P) slightly increases under higher U_(F)and C_(F). U_(P) is determined by a pressure difference between theintermediate stream and both side streams (i.e., the diluate andconcentrate streams). The pressure difference results from the energyloss due to friction by shear stress, which is changed by viscosity,density, and velocity. Higher salinity brings higher viscosity anddensity, which lead to an increase in pressure drop traveling along thechannel. The flow distribution is changed with an application of currentflux (FIG. 16 ). The average flow velocity of PM-flow, U_(P), for thediluate stream (U_(P,D)) increases with an increase in the current flux,whereas U_(P) for the concentrate stream (U_(P,C)) maintains initialvelocity. The increasing tendency of U_(P) decreases as U_(F) and C_(F)increases. This change can be simply explained by electro-osmotic flowthrough the negatively charged polycarbonate porous membrane. Firstly, alarge potential drop promoting the electro-osmotic flow is formed acrossthe porous membrane in contact with the diluate stream, but highervelocity and feed salinity reduce the potential drop due to a decreasein the influence of electro-osmotic flow.

FIGS. 17A-C shows the salinity variation at the outlets of the diluateand concentrate channels and at the end of the intermediate channel. Wemonitored the real-time salinity changes for a fixed flow velocity forthe outlet and current flux to achieve a constant outlet salinity (FIGS.18A-18C). All cases stabilized within 10 minutes, but a slower averageflow velocity of the feed, U_(F), requires more time to stabilize than ahigher U_(F). A higher U_(F) shows a symmetric salinity profile change,but a lower U_(F) shows an asymmetric salinity profile change with adelayed increase in salinity for the concentrate stream, indicating saltprecipitation on the CEM. The salinity of the intermediate streamincreases with the salinity of the concentrate stream, indicating thatthe highly concentrated salt in the concentrate stream leaks into theintermediate stream. The leaked salt, however, returns again to theconcentrate stream due to the geometrical character of the RF-ICPsystem. A higher U_(F) shows a lower intermediate salinity for the samecurrent flux application and for the same salinity in the concentratestream. It can be explained in two ways. One reason is that a higherU_(F) helps to wash out the concentrated salt plug by hydrodynamicconvection. The other reason is that a higher U_(F) generates a higherU_(P) to prevent diffusion flux from the concentrate stream even thoughthe salinity is the same in the concentrate stream. With this result, wecan conclude that a higher U_(F) prevents diffusion flux from theconcentrate to the intermediate stream and helps to wash out theconcentrate plug in the concentrate stream. Without salinity variationin the intermediate channel, there is no significant difference from thepreviously reported Tri-ICP desalination system [9] such that a higherflow velocity requires more current flux to achieve a certain salinityin the diluate stream, showing the symmetric salinity change for thediluate and concentrate streams.

Based on the salinity of each stream, the resistivity and powerconsumption of each stream is calculated in FIGS. 19A-19C and FIGS.20A-20C. The resistivity of the diluate stream exponentially increaseswith an increase in current flux due to the relation between salinityand resistivity (FIG. 21 ) [15], while the resistivity of theintermediate and concentrate streams decrease due to the increase insalinity. The power consumption of the diluate stream is notsignificantly higher than other streams under a lower salt-removedcondition, but most of the power consumed by the diluate stream under ahigher salt-removed condition is due to its rapid increase inresistivity. In some cases, the power consumptions for the intermediateand concentrate stream decrease rather than increase due to theresistance decrease.

2.2 Comparison of RF-ICP and Tri-ICP Desalination

We performed both RF-ICP and Tri-ICP desalination experiments with thesame spacer and membrane dimension to evaluate an improved performanceof the RF-ICP desalination (details of experiment set-up are given inFIG. 13A-13B). FIGS. 22A-22C shows the effect of the systems on currentutilization (CU), calculated from the experimental results, with respectto the feed salinity (C_(F)) and the current flux. Firstly, the CU ofboth systems gradually decreases as the current flux increases for allC_(F). The higher current flux leads to a thicker depletion andconcentration layer on the opposite side of CEM. The development of atrans-membrane concentration difference promotes stronger back-diffusionand osmosis, resulting in the reduction of CU [16]. Both systems show ahigher CU value under a higher U_(F). A higher U_(F) facilitates anincrease in CU by the improved mass transfer, reducing the thickness ofthe depletion layer. The difference in CUs between the differentvelocities tends to increase as the current flux increases. It indicatesthat the hydrodynamic convection has a more significant role at thehigher current regime. Second, RF-ICP shows a higher CU than that ofTri-ICP. Even though both systems have the same outlet flow velocity,Tri-ICP has a higher horizontal velocity component along the membranethan does RF-ICP. If we consider only the horizontal velocity component,the CU of Tri-ICP should be higher than that of RF-ICP, because a highervelocity improves CU. The configuration of RF-ICP facilitates thevertical mass transport by PM-flow and increases the traveling lengthwhere water experiences ion separation.

We plotted two graphs, the power consumption as a function of saltremoval ratio (SRRatio) in FIG. 23A. The two systems have similar trendsin power consumption, which require higher power consumption for higherflow velocity to achieve a certain SRRatio. However, RF-ICP has betterenergy efficiency than Tri-ICP in all cases, including salinity and flowvelocity changes. The difference in power consumption increases with anincrease in the SRRatio, indicating that RF-ICP has a better improvementof energy efficiency at higher SRRatios. The energy per unit ion removal(EPIR) was plotted as a function of the salt removal rate (SRRate) toevaluate the energy requirement for a specific mass transfer rate inFIG. 23B. SRRate can provide the removed mass per unit time, but SRRatiocan only represent the ratio of removed mass fraction (FIG. 24 ). LessEPIR is required at higher feed salinity and flow velocity to achievethe same SRRate. A large amount of ion with a high salinity solution iseasily transported to and through the membrane, and a higher flowvelocity increases mass transport near the membrane, reducing thedepletion layer.

2.3 Numerical Analysis

We also performed a numerical analysis using the previously developedmulti-physics ICP desalination model [17,18]. The magnitude of velocityis visualized in FIG. 25A. In Tri-ICP, the flow formed a parabolicprofile in each channel because the porous membranes worked as physicalwalls. In RF-ICP, however, the velocity of the intermediate streamdecelerates by outflux through the porous membrane, but the velocity ofother streams accelerates by influx through the porous membrane. Also,the PM-flow velocity increases linearly along the porous membrane,having the maximum value at the outlet of the channel. Both Tri-ICP andRF-ICP have effectively removed cations in the initial region of theCEM, where the ion depletion layer has a thinner thickness, showing goodCU and EPIR (FIG. 25B). Tri-ICP, however, creates a thicker depletionlayer along the membrane, rapidly deteriorating CU and EPIR, whereasRF-ICP develops a thinner depletion layer with good CU and EPIR. Thistrend is more apparent when a lower feed velocity generates a thickerdepletion layer. Generally, a higher velocity leads to an increase inmass transfer leading to a uniform current distribution along the CEM[19,20]. RF-ICP leads to a uniform current distribution, improvingdesalination performance, with the same feed velocity, but doubles therecovery rate. Interestingly, under the application of the same feedvelocity, RF-ICP has better desalination performance even though Tri-ICPhas a higher flow velocity near the diluate side of the CEM than doesRF-ICP. In the previous paper, it is proved that Tri-ICP with porousmembranes has a better energy efficiency than Tri-ICP without porousmembranes [9]. It has been experimentally and numerically shown thatRF-ICP facilitates an improved desalination performance, includingenergy efficiency and recovery rate, with a simple flow configurationchange using the same channel architecture.

2.4 Power Consumption Analysis

The performance of RF-ICP is evaluated to achieve a fixed SRRatio (10%,30%, 50%, 70%, and 95%) from various feed salinities and flow velocities(FIGS. 26A-26B). The details of the experimental setting andexperimental conditions are provided in FIG. 15 and in Table 2. Themaximum current flux was limited to 250 mA/cm². The harsh condition,requiring a current flux over 250 mA/cm², leads to malfunctions in theCEM, creating a confused concentration profile and a bubble formation inthe spacer. Higher power is required to promote pure and fastdesalination, which indicate a higher SRRatio and flow velocity,respectively.

Various feed salinities and the corresponding SRRatio can providedifferent applications. In the case of desalination with a feed salinityof 10 g/L, we can achieve a drinking level desalination with aconcentration of 0.5 g/L of diluate stream with a power consumption of33. 7˜61.7 Wh/L. Recently, several studies are reported to obtaindrinking water from low salinity brackish water (2˜3 g/L) with a powerconsumption of 0.8˜1 Wh/L [16,21]. However, they require benchtop (550cm² of membrane area) or plant scale (meter-long membrane length)equipment. RF-ICP still requires high power consumption, but it isapplicable to portable scale desalination with small equipment size (75cm²). The other application is a partial desalination of highly salinebrine (70 g/L). According to a recent batch-ED study, the powerconsumption has been reported to range from 19 to 21 Wh/L, desaltingsimilar salinity changes (90 g/L to 40 g/L) for multi-stage brinedesalination [22]. Also, the lab-scale Tri-ICP has required 5.6 to 213Wh/L to achieve 50% of the SRRatio with 70 g/L [9]. RF-ICP requires apower consumption of 23.2 to 49 Wh/L to achieve 50% of the SRRatio. Thispower consumption is quite competitive in value because RF-ICP is acontinuous process and facilitates an improved recovery rate from 25% to50% without re-circulation and reduction of membrane length from 30 cmto 15 cm.

2.5 Cost Analysis

Power consumption is an important consideration for a desalinationapplication, but it is much more important to calculate a total watercost, composed of capital and operating cost, to evaluate the validityof technology in a practical desalination application. In this section,we mainly focused on how an optimized water cost changes by the externalenvironment to treat a brine (70 g/L) and its application.

As previously reported [9,23], the simplified water cost model isapplied for water cost optimization (see section 1.6 for details). Themodel includes capital and operating costs to determine the water cost.The capital cost is mainly determined by the lifespan of the equipmentand its cost. An increase in the equipment lifespan, determined by thelifespan of the membrane, can reduce capital cost. Even the lifespan ofcommercial membrane is guaranteed for 10 years according to thespecifications provided by the membrane manufacturer [24,25], it iswidely known that ion exchange membranes have a lifetime of 4 to 10years while maintaining selective permeability, depending on the type offeed solution due to fouling problems [26-28]. The equipment cost isdominated by a membrane cost and its size in the electromembranedesalination. Especially, the membrane size is inversely correlated withthe energy efficiency. As the length increases, energy efficiencyimproves; therefore, operating costs are reduced, but capital costsincrease. The electricity cost is a key factor in determining operatingcosts. The selection of electricity cost is also crucial to optimizewater costs, because the electricity cost varies according to localconditions and is widely distributed between $0.025 and $0.325/kWh [29].In FIG. 27 , the water cost analysis was performed to understand howwater costs change with different scenarios, including equipment size,lifespan, and electricity cost. We import experimental data from theprevious reported model, Tri-ICP, which has bigger membrane size, 180cm², and better energy efficiency (RF-ICP has 75 cm² of membrane area)[9]. The water cost is optimized to the lowest cost value of the sum ofcapital and operating cost. The lifespan and electricity cost are set as5 and 10 year, $0.05 and $0.1/kWh, respectively. The optimized watercost for RF-ICP is $4.01/m³ ($1.69 and $2.32/m³ for capital andoperating cost, respectively), with a lifespan of 10 years and anelectricity cost of $0.1/kWh. It can be reduced to $2.57/m³ (0.85 and1.72 $/m³ for capital and operating cost, respectively) with a reducedelectricity cost of $0.05/kWh, albeit with the same lifespan.

The changes in capital and operating cost provide changes in water cost,but also system characteristics and their applications. The operatingcosts, determined by electricity costs, used for both RF-ICP and Tri-ICPincrease dramatically with an increasing SRRatio, indicating that allICP desalination is basically a power-intensive process, regardless ofmembrane size. Tri-ICP can further reduce capital costs by increasinglifespan due to differences in membrane size, although extending thelifespan of RF-ICP desalination plants has a relatively small capitalcost savings. For the case (SRRatio=70% and K_(E)=$0.1/kWh), Tri-ICPcould reduce the capital cost from $7.72 to $4.51/m³, maintainingoperating cost as $4.00/m³ with the increase in lifespan from 5 to 10years, whereby the water cost is reduced from $11.72 to $8.51/m³.However, in the same scenario, RF-ICP could hardly reduce the water costfrom $9.19 to $8.79/m³. This result provides that ICP desalination is asuitable technology for a region that has low electricity costs, whilethe smaller system ICP desalination is more sensitive to electricitycosts.

Generally, a coastal sea-water reverse osmosis (RO) process discharges65 to 85 g/L of brine waste to the coast [30,31]. The disposal tosurface water can save a wastewater treatment cost, but it causes anincrease in seawater salinity; e.g., seawater salinity around theArabian Gulf exceeded 40 ppt [32,33]. This higher seawater salinitycauses a significant marine environment problem because the salinity of40˜45 ppt appears to cause the death of exposed marine plants [34]. Thebrine wastes can be disposed of to a well or recycled by apost-treatment process, such as mechanical vapor recompression (MVR),which leads to additional cost ($4.7 to 18.9/m³ and $22 to 39/m³ arewaste treatment cost by evaporation pond and MVR, respectively) [35].This additional cost should be considered to be part of the desalinationcosts, and it accounts for the majority of the water cost calculationsbecause RO, ranging from $0.71 to $0.91/m³, has little change in cost[36].

In order to reduce total water cost, partial desalination by RF-ICP isapplied to reduce the volume of waste from RO. FIG. 28A illustrates anRO process incorporating an RF-ICP and waste-treatment process. Tocalculate the total water cost, including RO, waste treatment, andRF-ICP cost, we applied an RO cost model developed by Lienhard's group[37], fixed waste-treatment costs, and an RF-ICP cost model with alifespan of 10 years, and an electricity cost of $0.05/kWh. The feedsalinity (C_(F)) is set as a seawater salinity, 35 g/L, and the brinesalinity (C_(RO,b)) from SWRO is fixed as 70 g/L. The brine (Q_(RO,b))is partially desalinated by RF-ICP, and the produced water (Q_(ICP,p))and brine (Q_(ICP,b)) by RF-ICP are recirculated into the RO inflow(Q_(RO,in)) and discharged to waste treatment, respectively. The feedsalinity for RO (C_(RO,in)) can change depending on the SRRatio of theRF-ICP desalination, but it is limited to 30 to 45 g/L, which can beapplied to typical RO [38]. In FIG. 28B, the total water cost varieswith the change in RO, RF-ICP, and waste treatment costs. As the SRRatioof RF-ICP increases, the RO recovery rate increases and Q_(ICP,b)decreases, resulting in savings in the total water cost. Additionally,the total water cost can be changed by the waste-treatment cost. Thecost analysis was performed to assess the feasibility of RF-ICPdesalination with changes in wastewater treatment costs (FIG. 28C). Theincrease in waste-treatment costs dominates the total cost variation,but the increase in the SRRatio of the RF-ICP reduces the total watercost. The RF-ICP begins to show that it is cost effective whenwastewater treatment costs are greater than $3/m³. Above thewaste-treatment cost of $3/m³, the total cost decreases rapidly at thebeginning of the SRRatio increase and, when saturated, is at a certaincost regardless of a SRRatio increase.

2.6 Conclusion

Herein, we demonstrate the feasibility of an RF-ICP desalination systemby a portable-scale ICP RF-ICP device (75 cm² of membrane area). Theperformance of RF-ICP desalination was evaluated with various flow rates(0.83 to 6.67 ml/m per spacer) and feed salinity (10 to 70 g/L)conditions to achieve fixed SRRatios. The RF-ICP desalination achieveddesalination of drinking water with a concentration of 10 g/L. TheRF-ICP desalination was applied with two cost-analysis scenarios forpartial desalination of brine with a saline concentration of 70 g/L. Thefirst scenario was evaluated using an optimized water cost variation forICP models with different membrane sizes by lifespan and electricitycost. The RF-ICP desalination can achieve a total water cost of $2.57/m³under conditions with $0.05/kWh and a lifespan of 10 years to reduce thefeed salinity from 70 g/L to 35 g/L. The second scenario was a RO costvariation, which included wastewater treatment costs. The RF-ICP wasapplied to reduce the volume of waste and was cost-effective when thewastewater treatment cost is higher than $3/m³. ICP desalination is anew ion-separation process in the field of electromembrane processes.This study indicates that the ICP desalination technology can achieve animprovement in energy efficiency and recovery rate by applying awell-developed technology in the ED field to the ICP desalinationtechnology. While a similar idea can be applied to conventional ED(RF-ED), we validated the idea in the ICP desalination process.

In summary, in electromembrane desalination processes, such aselectrodialysis and ion concentration polarization (ICP) desalination,ion-depleted boundary fluid layers constitute the desalted, productfluid stream, yet they also result in high resistivity and loweredenergy efficiency. Manipulating fluid flow streams directly (e.g. usingspacers) is a new and under-explored method to break this fundamentaltrade-off for electromembrane desalination. In this work, a novelelectromembrane desalination architecture was studied that allows a feedstream to return to the feed direction (hereby named, return-flowarchitecture) to improve energy efficiency by limiting and controllingthe size of the depleted boundary layer, even at high current values.The technical feasibility of this idea was examined in an ICPdesalination process (RF-ICP) with a wide range of feed salinity from 10to 70 g/L. Brackish water (10 g/L) can be desalinated into potable watersalinity (0.5 g/L) with a small-sized device (75 cm² of effectivemembrane area) at an energy efficiency of 33.7 Wh/L. For partialdesalination of 70 g/L brine down to 35 g/L, RF-ICP desalinationachieved an overall water cost of $2.57/m³ ($0.41/barrel). These resultsshow that the return-flow architecture can improve the performance ofelectromembrane desalination, enabling more flexible water treatment formany applications.

Example 2: Counter-Flow Ion Concentration Polarization (CF-ICP)

ICP desalination is created by employing a unipolar membrane system(e.g., only using CEMs), which will create two zones (brine anddesalted) within the channel between the membranes. In ICP desalination,separation of brine and desalted flow is achieved by the fluidic splitat the end of the system. A high-flow scale-up (shown in FIGS. 29 to 33) can be achieved by stacking CEMs in a similar manner to conventionalelectrodialysis (ED). FIGS. 29 to 33 compare the bifurcate ICP (Bi-N)with the CF-ICP (referred to as “Bi-C” in FIGS. 29 to 33 ). Theconcentration profile, current utilization, and power consumption weremeasured, as described above in Example 1.

To evaluate the feasibility of Bi-C for desalination, concentrationchanges in diluate and concentrate streams for both systems (Bi-N andBi-C) were measured under an application of current flux (FIG. 30 ).Both systems show symmetrical changes in concentration profiles, andBi-C provided an improved desalination performance under the samecurrent flux application.

As the current flux increases, the concentration difference between thediluate streams of each system becomes larger, which means that the Bi-Csystem can remove ions more efficiently at higher current fluxes. FIG.31 shows changes in current utilization, feed velocity, and currentflux. The CUs of both systems gradually decrease with the increase incurrent flux. Also, the CU increases with the increase in feed velocity.Bi-C shows an improved CU as compared to Bi-N for all cases.

The concentration difference between the outlet of the concentratestream and the stream across the CEM beside the concentrate stream wasobtained to estimate the effect of trans-membrane back diffusion, whichreduces desalination performance (FIG. 32 ). The concentrationdifference of Bi-N is higher than that of Bi-C. The higher concentrationdifference can result in higher trans-membrane back diffusion, whichreduces desalination performance.

In FIG. 33 , we obtained the power consumption to evaluate the energyefficiency of different ICP spacer architectures (Bi-N, Bi-C and RF).The RF-ICP and Bi-C showed higher energy efficiency than Bi-N,indicating that the presence of return flow motion improved energyefficiency. With respect to Bi-C and RF-ICP, Bi-C has higher energyefficiency than RF-ICP. These results show that the reduction of theconcentration difference across CEMs results in an improved desalinationperformance.

Example 3: Comparison of Power Efficiency by Various ICP ProcessArchitectures (Spacers)

We have introduced various spacers, Bi-ICP (bifurcate normal flow),Tri-ICP (trifurcate ICP), RF-ICP (return flow ICP), and CF-ICP (counterflow ICP), for ICP desalination (FIGS. 9A-9D); and their unique featuresare summarized in Table 3. In order to evaluate the energy efficiency ofdifferent ICP process architectures (which differ by the kind of spacerdesign/engineering that is used), we obtained the power consumption as afunction of the salt removal ratio (SRR) with 70, 100, and 160 g/L offeed salinity in FIGS. 10A-10C. The original ICP architecture, called“Bi-ICP” (FIG. 9A), is able to separate and collect two streams—diluateand concentrate streams. Bi-ICP requires the highest power consumptionamong the spacers, given the same conditions. Because Bi-ICPsimultaneously collects a large amount of streams, including a thickbulk layer and a thin depletion layer. Then, Tri-ICP (FIG. 9B)facilitates the collection of the thin depletion layer in the diluatestream but suffers from a reduced recovery rate. Tri-ICP significantlyimproves energy efficiency as compared to Bi-ICP, generating andcollecting a thin depletion layer and minimizing the depletion layerthickness. RF-ICP (FIG. 9C), using the same channel structure asTri-ICP, still facilitates the collection of thin depletion layers butincreases the effective channel length by simply changing the flow path.This change significantly reduces power consumption again, as comparedto the power consumption of Tri-ICP. Another important benefit of RF-ICPis that the depletion layer across the entire membrane length is moreevenly distributed, increasing the overall efficiency. CF-ICP (FIG. 9D),inspired by Bi-ICP, increases the effective channel length for thediluate stream but maintains the effective channel length for theconcentrate stream. By reversing the direction of growth of the twostreams, diluate and concentrate streams, CF-ICP reduced atrans-membrane concentration difference, resulting in a reduction inback diffusion and osmosis. Therefore, it was shown (FIGS. 10A-10C) thatCF-ICP achieves the best power efficiency of all four architectures,given the same feedwater salinity, flow rate, and membrane lengthconditions. While the techno-economic model of these ICP processespredict that the current level of power efficiency is suitable for manybrine management applications, we will continue to engineer the designof these space structure to increase the energy efficiency of the ICPdesalination:

-   -   Collection of the thin depletion layer,    -   Increase in the effective membrane length for the diluate        stream,    -   Reduction in the trans-membrane concentration difference, and    -   More efficient desalination at low-salinity conditions (at the        end of the process).

TABLE 3 Various ICP process architectures: Type Fluidic compartmentFeature Bi-ICP Two channels One inlet and two outlets One porousSeparate collection of diluate membrane and concentrate streams Recoveryrate = 50% Tri-ICP Three channels One inlet and three outlets Two porousCollection of thin depletion layer membranes Recovery rate = 25% RF-ICPThree channels One inlet and two outlets Two porous Increase in theeffective channel length membranes Recovery rate = 50% CF-ICP Twochannels One inlet and two outlets One porous Minimized trans-membranemembrane concentration difference Recovery rate = 50%

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While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

All references, articles, patent applications, patent publications andpatents are incorporated herein by reference in their entirety.

What is claimed is:
 1. A device for purifying and/or concentrating afirst water stream containing charged contaminants, comprising: a firstion exchange membrane; a second ion exchange membrane, wherein the ionexchange membranes are characterized by the same charge, wherein achannel into which the first water stream can be directed is definedbetween the first ion exchange membrane and the second ion exchangemembrane, wherein the channel is further characterized as having aninlet end and a return flow end, wherein the inlet end is where an inletis located, and wherein the return flow end is downstream from the inletend when the first water stream is directed into the channel through theinlet; and a first porous membrane, wherein the channel furthercomprises a first outlet and a second outlet, wherein the inlet and atleast the first outlet are located on the inlet end of the channel andare separated by the first porous membrane that traverses the length ofthe channel between the ion exchange membranes and terminates at areturn flow zone, wherein the return flow zone is a section of thechannel at the return flow end, and wherein the return flow end is atleast partially closed, wherein the ion exchange membranes and the firstporous membrane are configured such that directing the first waterstream into the inlet of the channel and applying an electric fieldacross the channel forms an ion depletion zone comprising a purifiedwater stream and an ion enrichment zone comprising a concentrated ionaqueous stream, wherein at least part of the first water stream entersthe return flow zone and forms a first return flow stream that flows tothe opposing side of the first porous membrane and the first return flowstream flows in the direction of the first outlet, and at least part ofthe first water stream, including the water and the chargedcontaminants, adjacent to the first porous membrane flows through thefirst porous membrane joining the first return flow stream, and whereinthe purified water stream is the stream directed to the first or thesecond outlet, and the concentrated-charged-contaminant aqueous streamis the stream directed to the other of the first and the second outlet.2. The device of claim 1, wherein the ion exchange membranes are cationexchange membranes.
 3. The device of claim 1, wherein the ion exchangemembranes are anion exchange membranes.
 4. The device of claim 1,wherein the first porous membrane is a non-ionic porous membrane.
 5. Thedevice of claim 1, further comprising a second porous membrane thattraverses the length of the channel between the ion exchange membranesand terminates at the return flow zone, wherein the second outlet islocated on the inlet end of the channel, wherein the inlet is locatedbetween the first outlet and the second outlet, wherein the inlet andthe second outlet are separated by the second porous membrane, whereinthe return flow end is fully closed, wherein each porous membrane has acathodic side and an anodic side, wherein the first outlet is located onthe cathodic side of the first porous membrane, wherein the secondoutlet is located on the anodic side of the second porous membrane,wherein the ion exchange membranes and the porous membranes areconfigured, when the first water stream is directed into the inlet ofthe channel and an electric field is applied across the channel, todirect the purified water stream to the first outlet, to direct theconcentrated ion aqueous stream to the second outlet, to direct at leastpart of the first water stream into the return flow zone, to form asecond return flow stream that flows to the opposing side of the secondporous membrane and flows in the direction of the second outlet, and todirect at least part of the first water stream adjacent to the secondporous membrane through the second porous membrane joining the secondreturn flow stream.
 6. The device of claim 5, wherein the second porousmembrane is a non-ionic porous membrane.
 7. The device of claim 1,wherein the device comprises a plurality of the channels.
 8. The deviceof claim 1, further comprising an electrode and a ground, each locatedexternal and parallel to the channel and configured to create theelectric field across the channel.
 9. The device of claim 8, wherein theelectrode forms a second channel with the first ion exchange membraneand the ground forms a third channel with the second ion exchangemembrane.
 10. The device of claim 9, wherein the second and thirdchannels are filled with an electrolyte solution.
 11. The device ofclaim 10, wherein the electrolyte solution is the first water stream.12. The device of claim 1, wherein the ion exchange membranes are cationexchange membranes, wherein the first porous membrane has a cathodicside and an anodic side, and wherein the first outlet is located on thecathodic side of the first porous membrane, the inlet is located on theanodic side of the first porous membrane, and the cation exchangemembranes and the first porous membrane are configured to direct thepurified water stream to the first outlet.
 13. The device of claim 12,wherein the second outlet is located at the return flow end.
 14. Thedevice of claim 13, wherein the second outlet is located on the anodicside of the first porous membrane.
 15. The device of claim 1, whereinthe ion exchange membranes are cation exchange membranes, wherein thefirst porous membrane has a cathodic side and an anodic side, andwherein the first outlet is located on the anodic side of the firstporous membrane, the inlet is located on the cathodic side of the firstporous membrane, and the cation exchange membranes and the first porousmembrane are configured to direct the purified water stream is directedto the first outlet.
 16. The device of claim 15, wherein the secondoutlet is located at the return flow end.
 17. The device of claim 16,wherein the second outlet is located on the cathodic side of the firstporous membrane.
 18. The device of claim 1, wherein the second outlet islocated at the inlet end of the channel, wherein the inlet is locatedbetween the first outlet and the second outlet, and wherein the inletand the second outlet are separated by a second porous membrane thattraverses the length of the channel between the ion exchange membranesand terminates at the return flow zone, and wherein the return flow endis fully closed.
 19. The device of claim 18, wherein the ion exchangemembranes are cation exchange membranes.
 20. The device of claim 18,wherein the ion exchange membranes are anion exchange membranes.
 21. Amethod of purifying and/or concentrating a first water stream containingimpurities by electrodialysis, comprising the steps of: directing thefirst water stream into a first inlet of a first channel and into asecond inlet of a second channel of an electrodialysis unit as a firstfeed stream and a second feed stream, respectively, wherein each feedstream comprises water and charged contaminants, wherein theelectrodialysis unit has an anodic side and a cathodic side andcomprises at least three stacked ion exchange membranes (IEMs), whereina first IEM and a third IEM have the same charge polarity, and a secondIEM has a charge polarity opposite to that of the first and third IEMs,and further wherein the second IEM is arranged between the first and thethird IEMs, wherein the first channel is defined, at least in part, bythe first and the second IEMs, wherein the second channel is defined, atleast in part, by the second and third IEMs, and wherein the firstchannel is on the anodic side of the electrodialysis unit, and whereinthe second channel is on the cathodic side of the electrodialysis unit,wherein the first channel and the second channel are each furthercharacterized as having an inlet end and a return flow end, wherein theinlet end of the first channel and of the second channel is where thefirst inlet and the second inlet, respectively, are located, and whereinthe return flow end of the first channel and of the second channel isdownstream from the respective inlet end when the first water stream isdirected into the first and second channels through the first and secondinlets, respectively, wherein the first channel and the second channeleach further comprise two outlets on the inlet end of the channels,wherein the first inlet is located between the two outlets of the firstchannel and the second inlet is located between the outlets of thesecond channel, wherein the first inlet is separated from the twooutlets of the first channel by two porous membranes, respectively, thattraverse the length of the first channel between the first and secondIEMs, and terminate at a return flow zone of the first channel, whereinthe return flow zone is a section of the channel at the return flow end,and wherein each return flow end is fully closed, and wherein the secondinlet is separated from the two outlets of the second channel by twoporous membranes, respectively, that traverse the length of the secondchannel between the second and third IEMs, and terminates at the returnflow zone of the second channel, wherein the return flow zone is asection of the channel at the return flow end, and wherein each returnflow end is fully closed; and applying an electric field across thefirst and the second channels, wherein the electric field causesformation of two purified water streams in the first or the secondchannel and formation of two concentrated-charged-contaminant aqueousstreams in the other of the first and the second channels; entering atleast part of the first feed stream into the return flow zone of thefirst channel and forming two return flow streams that flow to opposingsides of the porous membranes; flowing the return flow streams in thedirection of the outlets; and flowing at least part of the first feedstream, including the water and the charged contaminants, adjacent tothe porous membranes through the porous membranes, joining the returnflow streams; entering at least part of the second feed stream into thereturn flow zone of the second channel and forming two return flowstreams that flow to the opposing sides of the porous membranes; flowingthe return flow streams in the direction of the outlets, and flowing atleast part of the second feed stream adjacent to the porous membranesthrough the porous membranes, joining the return flow streams, whereinthe purified water stream is the stream directed to the outlets of thefirst or the second channel, and wherein theconcentrated-charged-contaminant aqueous stream is the stream directedto the outlets of the other of the first and the second channel; andcollecting at least one of the purified water stream and theconcentrated-charged-contaminant aqueous stream from the outlets of atleast one of the first and second channels.